A board of law library trustees may invest surplus funds in excess of one hundred thousand dollars ($100,000) or of the average annual expenditures of the library for the four fiscal years immediately preceding the investment, whichever is lesser, in the Local Agency Investment Fund pursuant to Article 11 (commencing with Section 16429.1) of Chapter 2 of Part 2 of Division 4 of Title 2 of the Government Code or bonds of the government of the United States or of this state. Bonds so purchased may be sold at any time in the discretion of the board. In computing average annual expenditures for the purposes of this section, capital expenditures for the purchase of real property and construction of a library building shall not be included.