House conferees reacted coolly yesterday to a new "bottom-line" offer by the administration in an attempt to end a bitter deadlock over potential costs of a new farm bill.

Despite that reaction, Agriculture Secretary John R. Block, who crafted the new proposal with ranking members of the House-Senate conference on the bill, said he was "optimistic" the House delegates would accept the plan.

The conference has been stymied for several weeks over the bill's costs, separated by a difference of about $680 million that the administration insists must be pared from the measure before it will be acceptable to the White House.

The new plan, achieved by slightly altering price support levels for dairy, wheat, feed grains, rice and wool, would add $253 million to the $10.6 billion Senate-passed bill that the administration supports.

Senate conferees quickly voted yesterday to go with the administration offer. But their House counterparts, still rankled over having acceded to almost $6 billion in cuts from their own bill, said they would study it and return with an answer next week.

Deputy Agriculture Secretary Richard E. Lyng told the conference the administration could give no more. "Anything more simply wouldn't fly," he said.

"You mean we have to eat this?" asked Rep. Tom Harkin (D-Iowa). "It is offered by the Senate side and there's no compromise to be made with it?"