You likely know how long it takes you to make a quilt ...start to finish. Take this into account for this one. I would not "discount" it either. I don't care if the blocks are already made. You make have to square them up etc which is more work than if you had pieced each one. Ask me how I know! Batting, backing, quilting and binding it all should be included in your WRITTEN AGREEMENT. It is a business transaction IF you choose to make it one vs a gift for family.
( I don't do either....). Get 50% non-refundable deposit which will cover your out of pocket expenses. Over guesstimate the labor charge. You can always true it up once the project is completed. Everyone has to be in agreement on this project or you will be shamed into giving in on price etc. I sew for me....no one else. I learned THAT lesson years ago! Don't believe most of us? ..search this Board for the nightmares!
sandy