I often wonder about this myself.
Of course, there is no way to accurately gauge these kinds of things, but I pretty much ALWAYS go to a local bar to watch the events. I don't have the ability to pay for the PPV (don't have a tv hooked up) and the one time I tried using UFC.tv, it fucked me over royally -- I paid for UFC 162 on ufc.tv, pre-ordered but ended up trying to watch it after the event, so I started it and for some goddamn fucking reason it thought the best thing to play was the slow mo replay of the main event. Literally the first thing I saw was Weidman playing basketball with Silva's head. So, instead of watching the biggest upset of the year, I got it ruined for me.
Anyway, that rant aside, I always notice a huge influx of people for the "big" ppv's (148, 167, 168 etc) at the bar and I always wonder if those people would usually be ordering ppv's, or if they're casuals who only watch the big shows.
Then there is streaming etc.
The amount of PPV buys =/= the amount of viewers, that's for damn sure.
"The common denominator of the Universe is not harmony, but chaos, hostility and murder." - Werner Herzog