It seems like giving oneself a fair return for one's labor is hardest when a friend is involved.
I think you have three options:
Givens: Friend is paying for all materials.
You are supplying skill, time, and tools.
1) Do it as "a labor of love" and for the experience - no complaints allowed after item is finished.
2) Come up with a set price - use suggestions given
3) Pass - and tell her that too many friendships have been wrecked by this scenario.