According to Dave Meltzer, GSP v. Matt Serra did 100,000 buys in Canada alone, and some interest metrics that have been highly reliable suggest the show did 550,000-600,000 buys. Combined with the excellent and disproportionate numbers in Canada (Canada usually does around 10% of the US number), this could be a 650,000 buy show, putting it right up there for all time UFC shows.
This is great news for a number of reasons. GSP will be on top for a long time given his youth and dominance, and he is the first of the new generation stars to prove he can draw on top.