Yahoo and Wrestling Observer writer Dave Meltzer is usually the best source for PPV numbers, and he dropped a little tidbit of info regarding the last two events into the Wrestling Observer this week. The UFC 136 buyrate was originally estimated to be a little higher, and the UFC 137 estimated buyrate has to be seen as a terrible number no matter what. Here's what Dave had to say (subscription required to view the WO):

The latest PPV estimates we’ve gotten from cable sources are about 225,000 buys for UFC 136 (Maynard vs. Edgar III) and 280,000 for UFC 137 (Diaz vs. Penn). I should also mention that sources close to UFC claim the 136 number is considerably higher and 137 is identical to our estimates, and that both numbers are likely to end up in the same ballpark.

Interesting that Zuffa is still saying UFC 136 did better than the estimates figure, yet they're cool with the UFC 137 number. Overall though, these are troubling statistics. While the UFC 137 number can be directly attributed to Georges St. Pierre not being on the card, and Diaz/GSP in February will likely do a huge amount of PPV buys, this proves that B.J. Penn's drawing power is all but gone (as is his career at this point) and that the initial response to Nick Diaz was tepid at best. As for the UFC 136 number - I think they've learned that their titles alone aren't as valuable as they think they are. Who is wearing them is all that matters.