I have to disagree with you both here guys. I think this will do pretty decent numbers. Historically, Bisping has always been a good PPV draw in the USA.
There's no actual evidence of that though.
UFC199 only did 320k buys, despite being on in American prime time with two title fights on the card (Hendo was also on that card).
And unless I'm missing something, that's the only PPV he's ever headlined. His headlining fights against Silva, Leites, Rockhold (the first time), Le, Kennedy and Belfort were all on free TV or Fight Pass. He headlined UFC120 against Akiyama but that wasn't a PPV either.
Same goes for Hendo. Last PPV he headlined was 161 against Rashad that only did 140k buys. UFC 139 v Rua did 290k. If you go all the way back to 2008, he did 325k buys in the main event of UFC 82 (against Anderson Silva).
All indications are they're building this card to appeal to UK and European audiences: the other names they've announced for the undercard include Mousasi, Struve, Magomedov and Manuwa. None of them are huge draws. The only Americans on the card so far are Hendo and OSP.
There's plenty of time for them to announce some bigger-drawing international fighters, though they're likely saving their big draws around that time for the following card: UFC205 card at Madison Square Garden.
Since the start of 2014 the list of fighters who've broken 500k PPV buys is
very short: Silva-Diaz and Jones-Cormier are the only 500k+ cards that didn't feature either McGregor or Rousey (and UFC 200 - haven't seen numbers on that yet but it's reasonable to assume it was very big). Go back a few more years and it was only really GSP, Silva and Jones that could get those numbers.
TLDR: respectfully, I'll eat my hat if Bisping-Hendo gets 500K PPV buys, even in American prime time. It's not realistic.
A $2m+ gate I'll buy as realistic, though they'll make that a bit hard on themselves if they insist on putting the card on in the middle of the night local time...