Mike Fagan over at Bloody Elbow (who is an excellent MMA handicapper) already has the betting odds posted for Brock Lesnar vs. Alistair Overeem:
The fight announcement is hardly an hour old, and Bodog already has a line up for the December 30 clash between former UFC champion Brock Lesnar and former Strikeforce champion Alistair Overeem. Bodog has Lesnar and Overeem as a pick'em, and you'll need to lay -115 on either side to account for the vig.
Given concerns about how Lesnar reacts to getting punched in the face, I expect the early action to come in on Overeem.
Given the skill disparity in terms of the dimensions of the game (Overeem standing, Brock controlling on the floor), I find the notion of betting on either man to be deeply problematic. Both carry such strong skills and liabilities, betting is ultra risky here.
Also, Bookmaker has Overeem -135/Lesnar +105. So if you want Brock, head over there.