Well, I may as well put in my 2 cents worth (I'm sure it's really worth more than 2 cents though - ha!).
Anyway...I charge $10.00 to transfer each photo onto the special fabric. Then I make a guesstimate of charge based on how difficult the design is and if I have made it before.
When I just Long arm I charge .03 per sq inch for custom work. All this is figured in and somehow I pull a price out of my _ _ _. Sometimes I make money and sometimes I get burned. But...I love what I do and if I can get a little pocket money - that's a plus.
Be smart though and write out a receipt (friend, relative, or stranger) and get at least a 50% deposit. That way, they have all the info in writing and that's your chance to make sure all info is correct and your time frame is there. I always say 6 to 8 weeks. There is a certain amount of pressure working for someone else.
I say, 'go for it'! Commit to one and see how it goes. Good Luck!