According to Bryan Alvarez in Figure Four Weekly via The Underground, current estimates put UFC 117's PPV buys in the 600,000 range. This was where most higher-end guesses were in the days before the fight (based on the theoretical maximum business of an Anderson Silva fight), but it's mildly disappointing in light of how other indicators were going on the day of the show. The post at TUG has a good explanation of estimates vs trends:
Trending numbers are based upon pre-fight indicators (e.g., live gate, television ratings, or Google search frequency) that are known to have a significant correlation with PPV buy rate numbers. When a fight is said to be "trending" towards a particular number the analyst is essentially saying that the pre-fight indicators are currently at levels that would suggest the PPV buyrate will be X. Trending numbers do not incorporate any sort of actual number received from a cable company.
Estimate numbers are based upon actual reports from cable companies. These typically take a little longer to report, because the cable companies need time to gather the purchase data from their systems. Then, once reliable base numbers have been gathered, the company will extrapolate those initial figures to give an estimate of what the overall card has likely produced from a buyrate perspective.
Based on the legend of this fight and another strong hype job, I suspect that a rematch could easily hit 1 million buys. I don't think it could hit UFC 100 levels, but it wouldn't shock me if it ended up as the biggest Lesnar-free UFC show in history.