It is something many of us face on a regular basis. Also, the "hey you sew don't you?". I tell people I have to know them for 10 years before I make them a quilt and that I quilt but I don't sew. I don't do hems or repairs or curtains or anything else that would be "easy" for me to do.
I'll do a bit of looking, we had a good thread recently that included links from sites that make t-shirt quilts for sale and what they charge.
The big thing is to price everything you will need. I mean everything. Needles. Thread. Any stencils or quilting designs or markers. Fabric. Interfacing. Price it out and write it down. Then decide on what it is worth to you, some people use twice the cost of materials as a starting point but when people try to hire me, I explain that with 40 years of experience I am surely worth the $15 an hour minimum wage in Seattle and that a quality quilt takes me about one month of effort. Even a basic quilt is at least 40 hours. They usually don't go past that point.
I think I could easily sell my queen sized quilts for $350-500. My costs are generally under $100. It's not worth it to me to sell knowing that I am only getting pennies for my efforts. However, I give freely and make many quilts for friends, family and donations because it's what I do.
Edit: Here's that other thread:
What would be a "fair" price?