McDonnell Douglas Corp., responding to Navy Secretary John F. Lehman's threat to cancel its $40 billion F18 fighter project if the price of the plane does not come down, said yesterday that "the costs have been decreasing and are expected to continue decreasing."

However, in its two-page response statement, the company did not commit itself to pegging the F18 at a price of $22.5 million or less for fiscal 1984. Lehman has said the Navy will pay no more than $22.5 million for the plane, and has suggested more F14 fighters and A6 light bombers as an alternative. Both are made by Grumman Aerospace Corp.

McDonnell Douglas, in claiming its F18 is "far less expensive than the F14," cited fiscal 1983 flyaway prices of $25.1 million for the F18 and $38 million for the F14. Lehman, in his comparison, is using fiscal 1981 flyaway prices, which show a much smaller difference.

McDonnell Douglas, which teamed with the Northrop Corp. to build the F18, added that the plane, designed to be both a fighter and a light bomber, "is comparable in cost with the slower, less maneuverable, less survivable A6." It cited a price of $24.6 million in fiscal 1983 for the A6.

Airplane costs are heavily dependent on how many are ordered. The unit price comes down when big orders allow volume production. Lehman has said the prices of the F14 and the A6 would come down if they were bought in large quantities. But critics of that course warn that both planes need new engines, which would drive up the price.

McDonnell Douglas said buying F14s and A6s instead of F18s "would give the Navy reduced fighter and bomber capabilities while increasing total costs."

The F18 in its dual fighter and attack roles "has proved itself to be superior to the F14" and is "the most dependable and easily maintained, high-capability combat aircraft ever produced," McDonnell Douglas said.

The company noted that Australia, Canada and Spain chose the F18 after "long and careful examination of the alternatives," and said it "is making an unusually small profit" on the F18.

In another reaction to Lehman's threat to cancel the F18, Rep. Robert A. Young (D-Mo.) of St. Louis, where McDonnell Douglas is headquartered, called for "a thorough and complete review" of how the Pentagon is spending its money.