IT'S DISCOUNT MONTH on Capitol Hill. The cost of Congress has been slashed. Last winter Gerald Ford stung the lawmakers when he put a $1 billion price tag on the new, improved '78 model legislative branch. A billion dollars is a nice, round - and forbidding - number. So, by dint of a little trimming here and a little pruning there, the lawmakers have just managed to mark it down to something less awesome - $990 million.

True, the price has gone up from last year's $943 million, but everything cost most these days. There's been a congressional pay raise, and their workland is expanding so they had to add more staff. Besides, the "billion-dollar Congress" label is deceptive anyway. You see, that's the price for the whole legislative package, which includes the Library of Congress, the Government Printing Office, the General Accounting Office and - as a bonus - the Botanic Garden, too. Now, you could get the impression that these are arms of Congress, because congressmen have been getting so many free library books, free Congressional Records and free plants. But actually those institutions do about $300 million worth of work for other agencies and folks. Subtract that, and the cost of Congress itself - your basic, stripped-down model - is only $699 million or so.

You don't find that persuasive? Well, these fellows are thriftier than you know. You want greater economy, fewer extras, easier maintenance?Tell you what they're gonna do. They've just agreed to slash Congressional Record handouts in half. The Senate has decided to give up free shaving mugs, hair-brushes and shipping trunks. They're even going to put out the Congressional Directory only every other year. And because those sacrifices weren't enough to get the total bill under $1 billion, they arranged to redirect some library and printing revenues and cut appropriations about $30 million.

So don't worry about a $1-billion legislative branch. You can still get it here for just $990 million. And confidentially - just between us - it's still got a lot of extras (as standard equipment, of course). In fact, it's better than a bargain; it's a steal.