The Federal Register, the legal forum in which the government unveils its rules on everything from aircraft design to welfare recipients' income, has always been a kind of bulletin board, a convenient way for agencies to communicate with their constituent groups.

But the price tag on those bulletin board notices stands at $408 per page, and, with budgets being trimmed on all fronts, some agencies are beginning to question whether they they can still afford to post announcements.

The moves to trim these costs have a beneficial side effect as far as the Reagan administration is concerned: they are helping to cut the size of the Register, a decline that officials, including the president, have cited as evidence that the administration is making progress on deregulation.

Already the Interstate Commerce Commission, one of the biggest contributors to the Register, and the Federal Emergency Management Agency have sought less expensive ways of distributing information while continuing to meet legal requirements.

In 1982, the ICC paid the Federal Register $805,400 for 1,947 pages of rules and notices, including lists that let motor carriers, shipping companies, the freight forwarders who serve as travel agents for cargo, and other interested parties know who applied for licenses to enter the business.

The idea is to give advance warning so that people in the industry can warn the government about potential bad actors. The agency says it isn't abandoning that philosophy, but it has proposed saving money by putting the notices in a new "ICC Register."

Interested parties would have to pay up to $245 for an annual subscription, compared with the $300 the Federal Register costs annually.

According to ICC staff attorney Kathy King, more than 1,200 pages, or about two-thirds of the ICC's contribution to the Register last year, would be eliminated.

She said the commissioners are still considering comments on the proposal and expect to announce a decision in a month or so. Between 85 and 90 percent of the license applications that are posted go unchallenged, King said.

Over at FEMA, which works on everything from surviving a nuclear war to selling flood insurance, someone took a critical look at the roughly $2 million it spent one year on printing for the flood insurance program.

They figured that, since another book already lists so-called flood hazard areas, letting local governments, banks and real estate and insurance agents know if a particular parcel is eligible for federally subsidized flood insurance, FEMA could safely scuttle the Register notices.

Last year the agency could have saved $155,856 for 382 Register pages if the change had been made.

The Environmental Protection Agency's Pesticides and Toxic Substances Division last year considered doing something similar, eliminating the Register notices that told when a new chemical had been registered for use in pesticides.

But the agency instead decided to remove repetitive language to save space in the publication.

Because of the effort, said John Richards, who handles EPA's notices on pesticides and toxic substances, the number of pages devoted to these topics barely increased between 1981 and 1982, while the number of agency actions more than doubled.