i charge $20 an hour for sewing, no matter what i'm sewing; then you need to guess-timate your materials cost (remembering to add everything...fabrics, thread, patterns, interfacings...what ever you used. double or triple that amount, add in your time and you have a 'base' to start with and you can decide to adjust up or down as you feel is necessary. visit sites that offer handmade quilts for sale. remember as with anything location is a factor...what may sell in Chicago for $2500 may only sell in Northern Michigan for $250-$500
check out "quiltsforsale.com" and see what the designers there have been getting.
another way to figure it out would be to pay for an appraisal on the quilt then you can base your price on fair-market value and you will have documentation to show it's value.