Because he is a collector, he should at least have some idea of the true scope of work and time that goes into one quilt. I would charge no less than double the cost of supplies, and I would request the first half of that up front so as not to be paying out of pocket for supplies.
I have a friend who asked me how much to make a quilt for her DD's toddler bed. I am charging her $150 +shipping, which is cost of just fabric x2, not including batting (because I have a lot in my stash and she is my friend). I did let her know that she's getting it cheaper than if I'd been making it for a random person. So if you feel inclined to not to charge what you *really* think it is worth, tell him he is getting a special "friend" discount, and not to "advertise" that price.