I'm not familiar with their pricing structure, but that seems a bit high to me. If they can fix it, I assume it's not much more than getting inside that wheelwell which seems like easy access. I'd get a second quote to be sure.
I've always wanted to try the compressed air method, your dent seems like a good candidate. Might be worth giving it a shot as I don't think you can hurt it by trying!
Here is just one of the many examples on the web:
Compressed Air Dent Removal - YouTube
the only trick is to get a can with the right kind of chemical, do a little reading and you'll find what I mean.