Vince would offer him a lot, Brock returning would increase TV ratings massively and if he does do something like WrestleMania, PPV buyrates too. He was UFCs top drawer and look at their buy rate.
Many will pay to see Taker vs Lesnar.
The Rock has hardly improved ratings when he's been on, and please check how 'well' Survivor Series did with him on the card.
I don't know where you're getting the perception that Lesnar is a massive draw for them, if The Rock (a much bigger mainstream name than Lesnar) cant draw fans in, then Brock definitely wont. He was the top name in one of the worst periods in WWE history, akin to Bret Hart in 1995/6.
Brock is the reason UFC is a thriving genre of entertainment. He catapulted them and was their instant draw. UFC takes a hit without him, sort of like when Vince lost his guys to WCW but not to that extreme as they milked him to get a longterm FX deal.
Without doubt, he introduced a hell of a lot of wrestling fans to MMA, and was their undisputed biggest draw. But the difference between WWE and UFC (aside from legitamacy) is that while in the UFC, a star can be born out of one fight (Chael Sonnen vs. Anderson Silva), whearas WWE have to dedicate years of planning to get guys over (ie, Punk only really shot to stardom in 2011, his 5th year in the company, same for John Cena in 2005).
The UFC rely a lot less on just one draw, which is obvious considering the injuries, training camps and medical suspensions. They cant have a guy fighting 12 times a year........and that pays off for them, as their PPV's regularly get around 4x the buys of the WWE.
WWE is going to be launching a network soon, has more hours of tv to devote to him, he can make money off merch and licensing for his past there, and he would pretty much be covered medically by them if something went wrong. Plus they could always bring his wife in and give her a check too or even a HOF spot. They have a lot more options for Brock t this point of his life than UFC
I dont think its been stressed enough that the guy hates the spotlight. Strange, considering hes been two worldwide companies biggest star, but hes said as much in interviews. Hence why you dont see him on talk shows and such, the guy has said he likes to be away from all the publicity, thats why he lives on a damn farm in Minnesota and not in an L.A mansion. Plus, one of the biggest reasons he left the WWE was because of the travelling..
And if you want to compare buys and ratings, WM which is where he most likely would work out draws anything UFC has. And at the end of the day Raw craps on UFC in the weekly viewership department. SmackDown is also the best weekly cable draw on its night, which is the worst night for tv, and pulls in more viewers than UFC which comes on the most watched night of weekly tv, Thursday.
And like Dale and Mak said, Vince would pay. Short of Mayweather, Brock is the biggest drawing fighter there is. And he paid millions, not the kayfabed amount, but still millions, enoguh to make him put off a fight where he would have made more than Brock just pulled in, for Floyd when the business wasn't nearly as profitable and coming off the Benoit/Signature scandal
Wrestlemania is the draw, it will sell no matter who is on it, this has been proven. At this point, you could have Hornswoggle say he wants to break Takers streak, and people would buy. The Superbowl comparison is very true, doesnt matter whos playing, its been built up as an event that transcends the 'sport' itself.
As for the weekly TV thing, well when have the UFC ever had a weekly TV show comparable to your Raw's and Smackdown's?
They've had ONE show on Fox so far which had more viewers than anything WWE has achieved in a decade. The Ultimate Fighter is more of a Tough Enough/nXt vibe, and only done seasonally.
I have no doubt they could offer him a lot of money. However I don't think it would be enough for what they want from him. I very much doubt they would offer Brock Lesnar millions just for a one off appearance or short feud. They would want him to work full time (maybe allow him to miss house shows).