Dave Meltzer reports in the subscription only Wrestling Observer:

Early cable estimates, and granted, anything this early in the week can be way off, indicate about 850,000 buys for UFC 111 on 3/27, putting it in the top six for UFC of all-time. The number had to boost the spirits of the company coming off four straight shows that did disappointing numbers. Dana White didn't give a number, but in an exchange of messages before we got any figures, he indicated being thrilled with the early estimates he had gotten and that it looked to be well above my prediction (700,000) going in. If this number holds up, it would be the largest since UFC 101 on 8/8.

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The number speaks volumes for St. Pierre as a drawing card, where he has to be now be considered No. 2 in the promotion.
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An interesting note is that even with the big buy rate, the one hour of prelims on Spike only did a 0.9 rating and 1.2 million viewers, the lowest of all PPV prelims to date. The most recent airing of prelims, before the 2/6 show, did a 1.2 rating and 1.7 million viewers, which was the best. But that PPV after doing the biggest Spike numbers, a show headlined by Randy Couture vs. Mark Coleman, only did 275,000 buys, the company's lowest for a U.S. show in four years.

This shows more than anything that the UFC is a star-driven pay per view business. Georges St Pierre and to a lesser extent Frank Mir and Shane Carwin drew these numbers, not St Pierre vs the over matched Dan Hardy.