I think softbank isn't just a regular VC. My speculation is that it may provide other services like dealing with govt(s) that makes this offer even more compelling. Otherwise I don't see why GM would need an investment from softbank.
I think it's a lowball estimate, but for argument sake, let's say they're each worth $150k. At that price $2.25 billion will get you 15,000 robotaxis, and that's far from the only expense.
Waymo has announced a plan to deploy 20,000 vehicles in 2020, and that is what GM is up against.
You just can't have enough money in the bank if you're comitted to a scaled commercial robotaxi deployment and are planning for world domination.