This is a hard one to advise on. But, here goes. If you are to provide backing and batting, figure your cost. Then add on what it is worth for you to quilt it. since these are friends of yours, just give them the price. If they are really serious, they will pay it. If they want it done for free, you will get out of that. Don't do anything you will regret later when you are putting all that work into it. It is harder to work on other people's things than it is your own.