this is tough for a number of reasons. this is a friend, so you don't want to cheat her. but this is you, and you don't want to cheat yourself. you didn't offer. she asked. it's her fabric, but you had to work out the design details. this was something that should have been agreed on before the work was done. well, what is, is.
try this: figure out as near as you can, what you actually spent on products that you used. thread, binding, backing if you bought it, etc. that's a given. try to figure, as close as possible, how much time you spent. decide what your labor is worth. remember, you have the skills and she didn't. and the time you spent working on her items was time that you didn't spend on your own interests. multiply the hours times the price per hour and add the actual cost. that's the real price. the size shouldn't matter that much unless it gets soooo big that it's hard to handle. that would make the hourly rate go up.
if you want to discount because she's a friend, by all means, do it. you might charge only 75% or even only 50% of the normal price. how dear a friend is she?
you should set a going hourly rate in case anyone else wants you to work for them. whatever you charge will set a precedent. as your work becomes more in demand,you might raise your prices. but once the word gets out, and it will, people should know in advance what the approximate cost will be. no surprises later. and a deposit, please, up front.
as a professional sewer for many years, i can't tell you how many people wanted work done and never came back for it or paid me.