Another option would be to have a contract between you and the client. Basically the client would have to sign a piece of paper stating the price and whatever other conditions you have (i.e. a deposit or other payment arrangements, etc) before you begin the work. They would think twice before signing the contract because that's a legal document and you could then take them to court since they commissioned work from you. That would certainly stop the 'I can't afford this right now' hoping you would lower the price.