I think it's plausible that Lesnar doesn't show at MITB to cost him the match (it saves one of his dates) but rather, Heyman costs him the match.
Next week on Raw, Punk tells Paul Heyman to send a message to Brock Lesnar to tell him to stay away from MITB because he's prepared for him, knowing he might be there to cost him the match. Then at MITB, Punk looks like he maybe has the match won, but then, the briefcase starts to rise in the air as Punk reaches for it, ala Austin at KOTR '99. The match continues on, and Punk loses. The next night, Punk confronts Heyman in the ring. He tells Heyman he has his suspicions that Lesnar didn't raise that briefcase, he did, because screwing Punk over from the back in the control room just doesn't seem like Lesnar's style. As Punk continues to confront Heyman and maybe even put his hands on him demanding the truth, Lesnar's music hits. Lesnar walks down the ramp and gets on the apron and while Punk is staring him down telling him to get in the ring, Heyman low blows Punk from behind (a trick that's been done before, just look at Flair turning on Batista in May 2005) and that's when the truth becomes clear that Heyman was in on it all along. Lesnar slowly steps through the ropes as Punk is down on his knees and proceeds to destroy Punk, which includes bear hugging him and slamming him into the post on the outside. He drags him back into the ring as a lifeless carcass and then Heyman gets the mic and kneels down and reminds Punk that he has a rotten reputation for telling the truth but that he still fell for one of his lies as well.
This would make a great Raw ending.