Isn't hourly somewhat risky? What if you hire an inexperienced artist and it takes him/her a full 2 hours to do a finalized character concept that could have professionally been done in 1 hour? Wouldn't the math be more expensive than a bid?
Viewing post in Art freelance pricing
I guess it would be more risky... that's why I opt for the 'final product' option... I'm just mentioning that many artists DO use the hourly option, and I guess you have to examine their profile, and maybe ask in advance how many hours certain things should take... again I can only comment on what I found as the dev commissioning an artist. I hope some artists respond to your posts with their views.