If you will enjoy doing it..... Do what was suggested above, take the price of supplies and double it. (That's really what I do when I sell things..... I love knitting, and having an "order" gives me more of an excuse to knit more.
If you really want/need some money (which could be the case if you are a yarn addict as with a big addiction as I....... - I've come to the decision I'm never getting a credit card, or to never take on in a yarn store as I'm liable to buy them out.
) then charge the min wage, or decide how much time you put into it, and how much your time is worth to you.