According to the Wrestling Observer Newsletter, the UFC 88 PPV has done approximately 510,000 buys. This number is based on trade patterns, and not actual estimates. These approximations done at two weeks after a show are usually very close to the actual number.
The show had Chuck Liddell vs. Rashad Evans in the main event, where Evans knocked out The Ice Man. It also featured Rich Franklin beating Matt Hamill, but Liddell was clearly the draw here, thus making the estimate good news for Chuck.