Best case is essentially unlimited, e.g., $200-500 USD/hr. If you go on a project basis, the hourly rate could end up being very, very high. Or mediocre. Or even low.
Average? We have no idea what your capabilities are, so we can't factor that into the equation. Average is probably closer to $50-125 USD.
That's how I started billing clients initially; basically making stuff up, estimating project complexity v. time-to-implement, and so on.
Eventually they were reasonably accurate, then I stopped doing it for awhile. I've toyed with the idea of getting back into it again, but I'm a terrible hustler, and don't have the patience for it :(
I've always been curious about how freelancers make such a high hourly, because freelance websites are full of people charging $15/hr.