Personally I see this as a big lie.
Firstly BMW Sauber have openly stated that their goal for the next 2 or 3 years is not to win the championship, but to learn and improve. In fact the planned extensions of the Hinwil plant haven't even been started. I recall that Mario Thiessen said that even next year they are only going to start striving for podium finishes, but are definitely not in the hunt for the world championship.
On the other hand, there is Michael Schumacer who is getting way too old and is on the verge of retiring. He has openly stated that the only reason he is going to stay on in F1 is to win the championship one more time. Basically if MS and BMW join together, there is no chance that MS will win the championship because BMW is in a learning phase, and given his age, when BMW really do start to become front-runners, MS will be pretty much be past the peak.
Besides, its not like MS needs the money. Rumour has it that his latest contract with Ferrari is worth $60 million, not including sponsorship and endorsements, which would in total be worth around $200 million (don't hold me to those figures, I'm not too sure). On top of that his income is taxed in the lowest bracket, because in switzerland (where he lives) he is considered unemployed since car racing isn't a recognized job.
Plus it is said that he doesn't spoil his children at all, and wants them to learn the value of money, I read somewhere that he only gives them like $10-20 Euros pocket money a month. Although supposedly he has a go-kart track at the back of his house. So its not like he's got a use for it
(not that, that has stopped people in the past trying to get richer)