if you're a 2:21 guy and you have the type of form that translates to trails well, then you could probably get some shoes and gear, and probably some travel stipend, but you won't be a top trail runner unless it turns out that you are really good at technical trails (unlikely given your road background).
Stephen Kersh is a 64min half guy and would probably run 2:20ish for a marathon, maybe as fast as 2:16. He started doing some easier 50k ish races and was winning easily, then ran the Lake Sonoma 50 miler last weekend and got 4th, which earned him a spot into Western States. To be a sponsored ultrarunner, you basically just need to be fast enough to qualify for Golden Ticket spots.
But beware, there are plenty of ~2:35 guys who will beat you in the tougher trail races. 2:21 speed only helps so much when you have to dance down a path of rocks and sticks down the side of a mountain, and then hike back up.
Also, I'd wager that Jim makes about the same amount or more than the NAZ Elite team, although they all get appearance fees at marathons when Jim will just get free travel and entry. There are some races in China that will pay appearance fees, but that's about it.