If the CONTRACT says that it is only valid if work is performed in 30 days, you could terminate it and get your money back. But if it was the proposal only that had that language, not the contract, then what that meant was you could only get that pricing if you signed the contract to do the work within 30 days--but once you signed the contract, the contract supercedes the proposal and you are obligated to it unless the other side (the contractor) violates a material (important) term of the contract or the contract contains a provision allowing you out of it. So you need to look not at the proposal, which was intended to get you to signt the contract, but at the contract itself and only to see what your rights are. Contracts are enforceable as per their plain terms, regardless of prior proposals or discussions.