Last week, after a bit of unseemly wrangling over small change - a paltry $3.15 billion - the House finally approved a target federal budget of $500.9 billion for fiscal 1979.

Now, there are places in the country where $500,900,000,000.00 seems like Serious Money. There are places in the country where $500,900,000,000.00 is being discussed at great length.

They are talking about it in Kansas, for example, where the word "budget" is normally applied to the price of iceberg lettuce. They are talking about it in Illinois, where the word "deficit" means "debt," which means creepy calls from collection agents who tend not to believe that the check is in the mail.

But were people chatting about those billions in our nation's capital?, you ask. No, I answer. The major subject of conversation I heard here the day the target figure was approved can be written with two little words: housing costs.

While 80 percent of the population of this town could not guess how many pennies there are in the HEW jar, 99.9 percent of the people seem to know that a 7-foot-wide house in Georgetown sold this spring for $111,000.

So, like a lot of other visitors, I've become convinced that if Washington seems out of touch with Real Life, you can blame it on Real Estate.

When the average Georgian here writes home complaining that he cannot find a decent house for $100,000, who do you suppose believes him? And when some guy in government starts thinking that a $200,000 six-room detached house is a bargain, then a defense budget of $127.4 billion starts sounding perfectly reasonable. Think, after all, how many people can sleep in an MX missile base. Think of all the bathrooms, the closets.

Washington has finally become the winner of its own Golden Fleece Award and the rest of us are losers.

This is the city where street vendors sell quiche Lorraine, where a YMCA membership costs $750, where nursery school for a 3-year-old can run $875 a year and where a gallon of gas can cost 80 cents.

Never mind all the reports that Washingtonians have the highest disposable income of any of the top 10 metropolitan areas. The point is that their income is more quickly disposed of here.

In the pre-Unreal Estate days, an out-of-towner could go to a party here where everyone would be speaking in the foreign language of numbers and initials. You could spend a whole evening wondering how the "Title II" was doing with "H.R.460" at the EOB. Nowadays, however, you can spend the evening both uncomprehending . . . and underdressed.

This city that used to be dowdy is now living up, not to the Joneses, but to the property values. it is difficult, after all, to wear $20 blue jeans in a $100,000 house, and impossible to shop at Penney's with such an impressive mortage.

Furthermore, this city may have more working wives than any of the other 10 major metropolitan areas. But half of those wives are working for their real estate while the other half are working in Real Estate.

Washington in 1978 reminds me of New York in 1968. This is where people are coming to hustle. The difference is that most people come to Washington to make it and leave it. They want to turn over everything, from their house to themselves, at a quick profit. So the obsession here is with the real-estate market instead of the stock market.

But this is where the government sits and sleeps and votes and stores its sense of values. People here are so busy tabulating the rising rate of real estate at 20 to 40 percent a year that they can hardly understand the screams coming from the rest of the country about rates of inflation on anything else. And when a house here sells at auction one day for $350,000 and then sells again two weeks later for $550,000, who can worry about the federal budget?

You see, in Washington D.C., $500.9 billion sounds like just another down payment.