The proposed costs of running Congress and the rest of the legislative branch next year will be at a record level of $1,062,530,000, according to budget figures released yesterday.

It marks the first time that the fast-growing legislative branch will have topped $1 billion, an amount almost four times above what was spent just 10 years ago by Congress and its supporting agencies - the Library of Congress, Government Printing Office and General Accounting Office.

Slightly more than half the proposed legislative branch expenditures - $536,766,530 - represents the direct costs of running the House and Senate next year. That is just over the $1 million for each of 535 House and senate members that President Ford mentioned last week in tweaking Congress on its own failure to hold down spending.

An almost two-fold increase in congressional staffs over the past 10 years - to over 17,000 - has been the prime factor in the increased cost of Congress.

Next year, according to the projected budget figures, staff growth will continue but not at the same fast rate of the past.

For example, the amount set aside for House members' "clerk hire" - the people who staff their offices - is projected to grow more than 10 per cent or by $10 million, about twice the rate needed to meet cost-living increases for those already employed.

The amount for administrative and clerical assistants for senators is also to grow by 10 per cent or $4 million.

Committee staff budgets in the House and Senate, on the other hand, show only slight increases - both below that needed to meet the 5 per cent approved cost-of-living increases.

According to Capitol Hill aides, the freezing of committee growth reflects a sensitivity to recent criticism and a reflection of moves in both houses to cut back on the past proliferation of committees and subcommittees.

More than any other part of the federal budget, however, the legislative branch figures are subject to change, for they have not been reviewed. The House figures, for example, have been put together in the House Clerk's office. Its staffers have not consulted committee chairmen. The same is true of the Senate, where the Secretary of the Senate draws up the budget.

Thus when the committees organize and their budgets are drawn up, the present freeze could quickly be thawed. For example, the proposed budget does not contain funds being sought by the House Assassination Committee.

Both houses have their own computer systems, but neither has gone much beyond the experimental stage.

The Senate this year is expected to get a second computer - at a cost above $2 million - which supposedly will deliver an up-to-date report on the floor status of bills, amendments and nominations.

The House, which has been operating a separate computer system for several years, already has its own bill-status system on a computer, prepared with assistance of the Library of Congress. According to Senate sources, however, "the House system is a day late" and so the upper body designed a program for its computer.

The $6 million-a-year House Information System is expected to undergo basic change this year, according to aides at the House Administration Committee, which has authority over the computer setup. Those changes are not reflected yet in the House budget.

Among other increases projected in legislative branch agencies are:

A $5 million growth in the Library of Congress computer operation both to aid Congress and move to a fully-automated "machine-readable cataloging service."

An increase of $10 million on library distribution of books to the handicapped.

An allocation of $15 million to purchase a new site for the Government Printing Office.