Buy-ons?

[JayBird]

If a national act was touring and they had "buy-on" slots available, what are some things to look out for and what are the benefits of buy-ons spots?

I've heard that OzzFest runs the second stage this way and wondered what a resonable price would be for a smaller national with buy-on slots available...not for OzzFest, but in general...lets say an 8 week tour or so.