Silva-GSP would easily break 1 million PPV buys. The real question is whether or not it will be the best-selling in UFC history. I think it would easily be top 5, and there's a good chance it would be #1. Think about it, can you even imagine a single MMA fan who WOULDN'T watch that PPV? If you're not willing to pay for GSP-Silva then you just aren't willing to pay for any PPV.
As for casuals, I think even casual fans know who Silva and/or GSP is and they will be easily hyped by the inevitable Countdown shows and endless highlight reels of both legendary fighters. Dana would hype the sh*t out of the event, put it in Cowboys stadium or something, and rightfully call it the biggest fight in MMA history.
So yeah, I would expect it to be huge but realistically it would have a floor of 1.1 million and a ceiling of 1.8 million. I know it's a big range but there are a lot of factors involved, including the date and venue and other fights on the card.