Just how much money does it take to elbow your way in between Apple and Google in the mobile market? Remember, you’ve also got to beat back RIM while you’re at it, and hope none of those three develop anything you didn’t expect. Got a number? Is it in the billions? That’s what Microsoft may be betting to make Windows Phone 7 work.

According to TechCrunch, Microsoft could spend into the billions on development and marketing for the new mobile platform. It’s a huge figure, made to look even larger by the estimated expense of marketing the Droid series of Android phones.

The $100 million Goldberg estimates that Verizon, Motorola and Google collectively spent on marketing helped turn the Droid line of phones into a serious stable of competitors against the iPhone. (Compare that to Google’s fizzled Nexus One launch, where the search giant pinched pennies on marketing.)

To spend 10 times that amount could be either a great idea or the world’s worst gamble. Microsoft has completely scorched consumer bridges with its previous mobile offerings, none of which will transition to the new system very smoothly. Even with a billion dollars behind it, Windows Phone 7 could be too late.