I have 2 sugestions:
1: Don't!!! Mixing friends and money usualy isnt the best idea. If you do, be carefull.
2: Stop by Etsy
and take a look at the knit goods to get a kinda feel for what people are charging for what. Also, for the people that you dont want to knit a scarf for but are still asking, you can point them there to get one.