In tonight's finale of "Joe Millionaire," Evan Marriott selects his would-be mate and reveals to her that he isn't the moneybags he's pretended to be. So far, the deception has been good business for Fox, which airs the hit "reality" series. The network's asking price for a 30-second commercial on tonight's show is $550,000, one of the highest ever for a regular series.

And once the episode ends? Well, nice knowing you, Joe.

The series -- in which Marriott, a construction worker, posed as a millionaire to fool 20 gold-digging suitors -- was a one-shot concept, designed for just seven episodes. That means "Joe" won't have the lucrative afterlife of a conventional TV hit -- no reruns this season, no new episodes the next. There's also little chance Fox can resell "Joe" to local stations for endless repeats, much as it has done with "The Simpsons" or "The X-Files."

As reality shows crowd the airwaves, the success of shows like "Joe Millionaire" has begun to look like a mixed blessing to people in the TV business. There's little doubt that the reality genre has rewritten the economic textbook of network television, which for years has relied on expensive scripted programs produced by Hollywood's studios.

The question is, will this rewrite have a happy ending?

On one hand, reality shows can be a network's dream. They can be churned out quickly, and cost about half as much to produce -- about $600,000 per hour -- as the typical new drama or sitcom, according to various network estimates. Reality shows don't require new sets, or stars who hold out for $1 million-an-episode salaries, as the actors on "Friends" successfully did early last year. And when they connect, programs like "The Bachelorette" and "American Idol" tend to be popular with the audience advertisers desire most: young women.

As a result, reality series offer something scripted shows do not: a quick profit. The networks usually lose money on the first run of their dramas and sitcoms, due to high initial costs (NBC, for example, pays Warner Bros. studios $13 million per episode of "ER," $10 million for "Friends," and $5 million for "The West Wing"). This deficit isn't recovered until the show has been rerun two or even three times -- and sometimes not even then.

"We can order eight episodes of a reality show and break even or even make money right away," said David Poltrack, executive vice president of research at CBS. "We don't put them on with a big deficit."

The trade-off, however, is that reality shows burn out much more quickly than conventional series. "Temptation Island" was a modest hit on Fox in its first season, but viewers disappeared when it came back for a second. On cable, "The Osbournes" and "The Anna Nicole Show" started strong last year, but have almost been forgotten now. "Who Wants to be a Millionaire" was a huge success on ABC when it debuted in August 1999, but tumbled within two years after ABC played it as many as four times a week.

The only reality show that has stayed near the top of the Nielsen ratings for more than three cycles has been CBS's "Survivor." Top-rated sitcom "Friends," by contrast, is in its ninth season.

What's more, even the most popular reality shows aren't very popular in repeats, which are critical to the long-term financial success of a TV series. A popular sitcom or drama tends to have a following for years after its original network run, and can churn out profits on the rerun, or syndication, market for decades.

Fox has tried to get extra mileage out of "Joe Millionaire" by broadcasting each episode twice each week, but has no plans to return the show to air after tonight's finale. ABC has milked its reality franchise, "The Bachelor," by repeating episodes on the ABC Family Channel, one of the cable networks owned by its parent, the Walt Disney Co. But even ABC officials concede this has been a limited success. No one expects "The Bachelor" or today's biggest reality series, "American Idol," to approach the most popular sitcom of yesteryear, "Seinfeld," in syndication. "Seinfeld's" producer, Sony-owned Columbia Tri-Star, collected more than $1.5 billion in 1998 from cable and local stations eager to carry reruns of the sitcom.

All told, "the economics of [reality] shows are not as great as people think," said Jeff Zucker, NBC entertainment president. "It's not the economic boom that people think it is. . . . This is all uncharted territory for everyone. The model is still evolving."

One advantage of dramas and sitcoms is that they tend to get fairly steady and loyal audiences week after week -- an important selling point for advertisers. Not so reality shows, where the lack of a suspenseful narrative makes it easy for channel surfers to flit in and out.

On Sunday night last week, for example, the audience for "Fear Factor," NBC's gross-out game show, surged by more than one-third in its second half hour; a smaller but similar surge occurred when the show aired on Monday and Tuesday nights, according to Nielsen audience data. Over the course of three airings during one week in January, another NBC reality show, "Meet My Folks," saw its audience decline from 14 million to 11 million to 8 million.

Advertisers recognize this, too, which is why the ad rates for many reality shows can be as much as 20 percent lower than for dramas or comedies with roughly the same number of viewers, executives said. Advertisers are leery, too, of some of the more extreme behavior and situations on some reality shows. " 'Fear Factor' is not a very advertiser-friendly program," noted CBS's Poltrack.

"Dramas and sitcoms are a safer environment" for advertisers, acknowledged NBC's Zucker. "They know the audience is probably going to be around next week, and maybe next year. We're better off finding the next big [scripted series]. The problem is, no one has done that very well lately."

In fact, no new drama or comedy series has consistently ranked among TV's 10 most popular since "CSI" debuted in October 2000. After "CSI" -- currently the most-watched program on TV -- all of the most popular entertainment series made their debut sometime during the 1990s: "Friends," "ER," "Everybody Loves Raymond," "Law and Order," "Will and Grace."

Hollywood's labor unions have their own negative review of reality TV. The unions are concerned about the trend because reality programs use nonunion performers and independent production companies that aren't covered by collective bargaining agreements. This not only takes away new work from actors, but also the fees -- known as residuals -- that actors and writers receive each time their show is rerun.

"It's a terrible trend, and very worrying to us," said Melissa Gilbert, president of the Screen Actors Guild. "My concern is that once the networks get comfortable with a certain kind of program, it becomes very tough to make a trend go away. . . . If we can't stop it, we can certainly try to organize it and oversee it."

Anthony Zuiker, the creator and executive producer of "CSI," predicted that "we've sort of hit the reality peak. I'm not sure this is a form that's getting better, not when you have to lay one gimmick on top of another."

But there's no sign that reality is going away soon. ABC last week launched "Are You Hot?," a show in which men and women compete solely on the basis of sex appeal. It has scheduled 10 more reality series through the summer. NBC is planning to carry "Around the World in 80 Dates," in which a contestant has romantic escapades with partners from other countries. CBS is readying "Cupid," a series similar to "The Bachelor."

Fox, meanwhile, has bought "Spellbound," a reality show in which three attractive young women are hypnotized to believe that an unattractive man is their perfect mate (the show is produced by Elisabeth Murdoch, the daughter of Rupert Murdoch, who heads Fox's parent company). And Fox will replace "Joe Millionaire" next month with "Married by America," a series in which viewers vote to match-marry contestants.

In all, as many as a dozen reality series of one kind or another could be on the networks' schedules next fall.

It's possible -- as CBS Television President Leslie Moonves recently conceded -- that the networks are overdoing it, that saturation will eventually turn viewers off. This is what seems to have happened to another kind of low-cost programming, newsmagazines, after shows such as "Dateline" dotted network schedules almost every night of the week.

Reality could even become a victim of its own success. Network executives say that the cost of renewing successful reality series has begun to creep up, as independent producers demand more money in a hot market. Even untried but promising concepts, they said, are getting more pricey as the networks bid for rights. "What used to be a $400,000 [per episode] show is now a $700,000 or $800,000 show," said one executive, who requested anonymity.

It's still less than making an episode of "The West Wing" or "24" -- but is it as smart?

"CSI's" Zuiker theorized that there will always be some place on TV for "escapist" fare, but people have longed to see and hear "well-crafted stories" since the dawn of time. "The big powerhouse dramas that people watch episode after episode will always keep the networks on the air," he said. "Those are the kind of shows that make it possible for network executives to eat cheese cubes on their yachts."

Hiring unknowns like Evan Marriott, Fox's "Joe Millionaire," helps keep the cost of producing reality shows much lower than dramas and sitcoms."The Osbournes": Last year's hit, this year's afterthought.