Ugh.. I have this SAME issue. The same with gestures.. I don't actively have a store, but I'm always being asked "Oh.. who made that gesture?" "oooh.. who did your profile picture?" I've been in SL a number of years now, so I tend to make my own gestures, shapes, and do most of my own photo work as well.
So how do you charge somebody for what you do because you ENJOY it, while at the same time making the lindens that it's "worth"? I always have to consider that I'm doing it for the joy of it.. not because I'm trying to pay tier (although tier is nice! ). I once had a very nice lady take a couples photo for me and my partner (who is now deceased iRL, sadly). She did an amazing job, but she was also working for a photostudio at the time. Now, I think I paid 1200 for that ONE picture (i got a 5x5 size and a profile picture size). Was it an amazing picture? Yeah.. it was pretty good. Yeah.. I could have done it myself.. but I was feeling lazy. Plus, it was nice to NOT fuss over critiquing my own work for once. That's why i was more than willing to ask her to do another one for me, and tipped her 600L on top of it... I was so thankful for the peace of mind. I trusted her.
Normally, I wouldn't advise paying that much for a photo, but really.. that's what most decent photographers in SL charge nowadays. Unfortunately, what about the little guys?
I don't know how much to charge for my own work, simply because I don't think it's THAT amazing. There's always room for improvement.
.... what a worthless response.