YOur idea is great if you really are able to keep up. My problem is that I have been doing this for so long that some of my stash is older than my child who will be 32 in November. I also tend to purchase things on sale just in case and anytime I have a coupon.
I recently made two t shirt quilts for a family in my community. I kept track of the suppies and fabric I purchased for just that purpose. I also tried to keep track of my hours. I ended up resorting to charging them by the square inch and adding my costs for the materials. I gave them the two quilts for $.10 a square inch. The quilts were very simply machine quilted with a little handquilting thrown in where I wanted to do more detail. I used flannel for the backs and didn't use batting as I had fused interfacing on the back of all of the shirts to stabilize them. Both quilts ended up very heavy even without the batting. While I was happy to get the little over $600 I charged them, realistically for the hours I spent it was worth double that. Be careful you don't end up feeling bad because others don't appreciate your work. The most important person to satisfy is yourself.