Usually when Sen. William Proxmire (D-Wis.) makes the news, he is flailing some example of government waste. But today he earns publicity for singlehandedly costing the U.S. Treasury at least $500,000 this month, a figure to which he is adding about $100,000 a day. As October goes on, the rate will rise even faster.

Proxmire is unabashedly trying to help the dairy farmers so important to his state's economy. Asked yesterday if he minded being held responsible for this drain on the Treasury, Proxmire replied:

"No, when you consider what's at stake here--after all, milk is such a vital food . . . . I think it's important to push this as hard as I'm pushing it."

So the inventor of the "Golden Fleece Award" commemorating what Proxmire considers egregious examples of government profligacy now is a contender for the prize himself, at least in the eyes of the administration, which is fit to be tied over Proxmire's legislative tactics.

But the senator has rejected all pleas from the White House to cease and desist. He even turned down a personal plea from Budget Director David A. Stockman.

Proxmire is taking advantage of a large crack between two legislative floorboards. During the fiscal year that ended Sept. 30, the milk price support was $13.10 per hundredweight and, during the new fiscal year, the House and Senate have formally agreed that it should continue to be $13.10.

But the two houses have not enacted a final farm bill for the new fiscal year and, when the old one expired Oct. 1, a previously enacted law automatically came back into force. It provided for a milk price support of $13.49.

The administration tried to pass an emergency bill to caulk the crack by maintaining the $13.10 level until the new law takes effect. The House has passed the bill, but Proxmire will not let the Senate consider it. Unanimous consent is required to bring up such emergency legislation, and Proxmire has objected to it three times. He promises to continue objecting.

Over a full year, according to administration and Congressional Budget Office estimates, this difference would raise the total cost of the dairy price support program by nearly $400 million. After dividing that number by 365 days in a year, administration officials said in late September that the Proxmire gambit would cost the Treasury more than $1 million a day, a figure reported in an earlier installment of this series.

Proxmire and his aides protested correctly that the figure was inaccurate. Because October is the slowest month of the year for milk production, the costs now are closer to $100,000 a day, although they are accelerating daily.

Moreover, any cheese, butter or powdered milk processed during this period will be redeemable at any later date at the higher price now being offered by the government, even after a new farm bill with the lower price comes into effect. If, as now seems likely, a final farm bill is not approved for another two weeks, Proxmire's gambit will have cost taxpayers perhaps $5 million or more.

"The cost," Proxmire said yesterday, "is relatively modest; they can handle that." The cause, he added, was the survival of the dairy farmer. If the administration-backed dairy program approved by the Senate is enacted, he said, dairy farmers will lose 70 percent of their net income, an "absolutely savage" reduction.

Common Cause yesterday released figures suggesting that other members of Congress also see political considerations in their attitude toward the dairy legislation.

When the dairy lobby beat the White House in a key House vote last week, 243 House members supported the lobby. Of them, 181 had received $1,037,784 in contributions, an average of $5,734 each, from the three big dairy political action committees during the 1978 and 1980 campaigns.

Of the 153 who voted against the dairy lobby and for the administration, only 68 had received dairy contributions, averaging just $1,616 each.

Yesterday, the House had a chance to reconsider that vote, and Common Cause had distributed these figures beforehand, apparently to try to shame members who received dairy money and then voted for the industry and against the White House.

The dairy lobby won a bigger victory, 255 to 153.