I wouldnt say that Brock is in it for a payday completely, he is, but he isn't. Started off in a small show, it was only one but at least it was a different show rather than jumping into the UFC right away.
Lesnar had no job after he quit wrestling in 2005, he is making some good $$$ in MMA oh yes, but he made more in WWE. WWE is global, they sell more merch and other crap like bail3yz stated.
As far as them showing WWE footage as pre-fight footage, I remember watching him powerbomb and land backbreakers on men that aren't exactly small in their own frame. It takes alot of power and energy to throw dudes around like that, that shit ain't easy, it may be fake but its definately not easy. And I think this is why that footage was included, not because they wanted to advertise WWE.
I don't think he took Mir seriously either, Lesnar struck me as the type of guy that wanted to get right into the mix and take over the HW division where ever he ended up. I came to this conclusion after his victory over that can back in June or July. "Lets get some titles on the line..." he said something like that, Yeah right chump, your going to get a title shot after one freakin victory. I also think that he has a hard time respecting anything that doesn't invloving using your huge muscles to perform IE Brazilian Jui Jitsu. "The Muscle Assholes" as George Carlin would put it. We all what these guys were like in High School, problem with Brock is, he still hasn't left High School.