I havent seen many sites doing banners for $500 and to be honest, theres no way I would spend $500 on such a small image...
A banner is simply one image with usually around 4 frames, which each might have a very small change in the image itself, but normally its just the text thats different.
I think its different for a logo though, a logo requires a lot more creativity than a banner I think and as long as it was VERY good, I would spend up to that price range for a logo.
Even if someone worked on a banner for you for 5 days, working full time, thats $100/day.
I dont know anyone that would take so long to work on a banner full time for that long.