There really is no formula for this, because some quilts take very little time and some take a lot of time. The only real way to price them is to keep track of your time and charge an hourly rate, plus the cost of materials and shipping.
A queen sized double wedding ring quilt should sell for much more than a queen sized Log Cabin quilt. If a quilt is machine quilted, it has taken weeks instead of months - it should be priced accordingly.
Lately, I have been pricing quilts with estimates for labor - telling my clients that they will need to pay for the materials up front and half of the labor cost - the other half of the labor cost and the shipping charge is due when I email to tell them their quilt is finished. This also helps them decide what they want to spend on fabric. They can decide if they want Walmart or the local quilt store - or I help them shop online for better fabrics at Walmart prices. They like the flexibility and they gain an understanding of what fabric really costs!!