Recently saw online a web site that advertised that the web site owner would make a quilt, start to finish, for $500 labor. That did not include materials!
Why not put one in the shop for $250 - $300 and see what happens? Be prepared, though, one thing that might happen is that your MIL will stop asking you to make them for her shop!
Also, ask MIL what she had in mind to charge when she asked you to make the quilts. She must have a target price/market in mind.
Good luck!