I think most people think that because we love our craft that we will just love making them something. If in your mind the request is a commission then your first question to the person making the request should be "how much money are you expecting to spend on this quilt". This should alert the person that they aren't getting a freebie. You then whip out a sheet that shows an estimated cost for each size quilt you make.
I don't quilt for money. When someone asks me about making a quilt for money I show them a lap quilt and say "it cost me around $300 to make that lap quilt". Usually they go quiet and don't mention it again. I think they are thinking I'm crazy and they can buy a quilt made in China for $49.99 at Macys. My sister has the same problem with people wanting a free painting.