if you are serious about taking on a (selling- market) you really first off need to get organized---and save reciepts!
if you are going to make money with your craft at some point you will have to pay some taxes= good records are a must- and can save you (TONS!)
if you are using your stash you already paid for your fabric- so what is the average you spend on your fabrics? do you shop at wally world and hit the $1 a yard tables? do you use only lqs fabrics and pay anywhere between $8.99 & $11 a yard?
figure out your average- and write it down==if that is the amount you think reasonable. lots of times we figure (about) 3 times the cost of materials- as a pretty fair starting point. some people decide their time is worth a certain amount (like i charge $20 an hour for sewing for people--like hemming- mending--home dec sewing)
when i am selling items i've made i do not bother figuring out if i used 6" of this or a yard of that- i kind of (guest-amate) the total amount of fabric used- it its a purse with 1 1/2 yards of fabric in it- then i use my average fabric price with that amount---i don't worry about individual---but i have all receipts for every piece of fabric used.
you need fabrics/materials receipts also if you are planning on using donations for tax purposes.