Some of the best advice I ever received was to view yourself as an investor first and an entrepreneur second. Everything you do is an investment, of time, money, energy, focus, or other currency. When you're considering what projects to do, where to spend your time, or how to spend your money; look at it as an investor would; ask yourself, "what ROI can I expect from this?"
Answer: understanding the concept of risk & reward will help you mentally accept a seemingly "overpriced" fee for your service. A client who agrees to a fixed rate lowers your risk and potential reward... You are also incentives to work as quickly as possible on the project (even if you have pride in your quality), every extra hour spent on the project reduces you profit. Agreeing to a performance based payment reduces the clients risk up front, they only pay if tangible results are reached, but this greatly improves your potential reward.