Sen. Lowell P. Weicker Jr. (R-Conn.), chairman of the Senate Small Business Committee, said the Reagan administration's plan to shut down the Small Business Administration is a "shortsighted effort" that would cost three times more than it would save.

Weicker's comments came in a letter to Senate Majority Leader Robert J. Dole (R-Kan.). Senate Republicans are working on an alternative to the president's budget and have sought ideas from Senate committee chairmen.

Weicker said Office of Management and Budget statistics indicate that $1.8 billion of the savings the administration projects would come from selling off loans in the SBA's portfolio worth $10 billion for about 19 cents on the dollar.

"Thus, a loan portfolio valued at $10 billion would be sold for less than $2 billion," Weicker wrote. "According to historical collection data at SBA, if the agency managed the portfolio it would collect approximately 80 cents on the dollar. By accepting OMB's solution, the government in fact loses some $6 billion."

"How do they know what the budget will propose?" asked OMB spokesman Steven Tupper. Tupper suggested that Weicker might want to wait for the proposal to be sent to Capitol Hill before he criticizes it.