(a) Public projects of seventy-five thousand dollars ($75,000) or less may be performed by the employees of a public agency by force account, by negotiated contract, or by purchase order.
(b) Public projects of two hundred twenty thousand dollars ($220,000) or less may be let to contract by informal procedures as set forth in this article.
(c) Public projects of more than two hundred twenty thousand dollars ($220,000) shall, except as otherwise provided in this article, be let to contract by formal bidding procedure.