I won't post links to any other commercial render farms, but a quick google will answer a lot of questions. I think render farms often price according to GHz-hours (i.e. you pay more if you use more cores and for longer). There can also be tiered pricing based on priority (rush job = moolah) etc etc.

I think the project files would have to be set up with relative references to textures etc so that the project folder is a standalone entity. I doubt David (though I don't want to speak for him) would enjoy going through someone's textures trying to link them all up. Render settings would most likely be set up already (frames and cameras etc) so that the rendering could be started from the command line.

Those are my two cents, sorry David if I've spoken out of turn.