Depending on the geographic location, quilts can be a really hard sell. Shoppers are looking for Walmart prices. I agree with the poster who said to estimate the number of hours in the project. Then realize that there's no way you will even get minimum wage
When you add in costs of materials, and estimate $75 just for the quilting, you are in the stratosphere!
That's why people just take the NON SALE cost of the fabric, batting & thread and double or tripple it.
I would provide the hours worked and the material and quilting costs to your DMIL, and tell her that unless she feels that she can get the big bucks, the large quilt will be a one of a kind, but you will love to make smaller quilty things, like placemats or totes until you see what sells. You might be pleasantly surprized!
If forced to come up with a dollar amount, I would think the 200 to 300 dollar range.