High coffee prices triggered mostly by Brazil's frost are mainly responsible for a 30 per cent jump in the cost of imported agricultural products, meaning that the 1976-77 U.S. farm trade balance will be the smallest in four years, the Agriculture Department said yesterday.

The total cost of agricultural imports for the year to end next Sept. 30 is estimated at a record of $13.6 billion compared with about $10.5 billion in 1975-76, the department's Outlook and Situation Board said.

Farm exports, however, also are expected to set a record of $24 billion this year, up from slightly less than $22.8 billion in 1975-76. Even so, the export increase will fall short of the rise in imports.

Thus, the next farm trade balance - the difference between the value of exports and imports - is expected to fall to about $10.4 billion this year from $12.25 billion last year. That will be the smallest since the balance was $7.24 billion in 1972-73.

The growth commodities this year in U.S. farm sales overseas include cotton, oil seeds (including soy beans, meal and oil), livestock products, fruits, nuts and vegetables. Grain exports, however, are down.

"Substantial increases are expected in fiscal 1977 in our exports to the Middel East, East and Southeast Asia including Japan, Western Europe and Canada," the report said. But farm exports to the Soviet Union and South Asia are "projected well below last year's levels," and sales to South America also are less, it said.