Although it appears to have solid support in Congress - and is a worthy cause - efforts to set up a trust fund to purchase park lands, open space, and historic buildings is too expensive at a time when the federal budget is running such huge deficits.

A measure approved by the House Interior Committee last week would create the American Heritage Trust and bankroll it with enough money by 1994 to produce interest earnings of at least $1 billion a year. Those earnings would be outside the usual congressional budget process.Certainly, it makes sense to acquire historic sites and other park land as quickly as possible. Every delay either raises the cost, or the property in question disappears into other uses.

Back in 1965, Congress authorized the spending of $900 million a year for such projects, with the cash to come from sale of surplus federal property, motorboat fuel taxes, and revenue from oil and gas leases on the Outer Continental Shelf.

But the actual outlay in recent years has been $100 million or less because of the budget squeeze. The money has been diverted by Congress into other programs and priorities. Both backers and opponents of the trust fund bill agree that there is a $3 billion backlog of desired but unacquired properties.

Despite these good arguments, the realities of the budget cannot be ignored. If money is pumped into a trust fund, billions of dollars must be taken from other projects, without lowering the deficit.

How much money would it take to produce a trust fund capable of earning $1 billion a year? Interior Department officials say $30 billion might be needed. Where can Congress find $30 billion in the next five years?

In addition, it would cost the Interior Department more money to develop and maintain $1 billion worth of new properties each year. Also, the interest payments wouldn't be "free money" because they would have to come out of the federal budget, too, since the trust fund would be invested in federal securities.

The trust fund is a good idea. But even though delay hurts, let's wait until the federal budget is more nearly balanced before embarking on such an expensive project.