50/50. I've done it where we've all stuck a couple/few hundred bucks in the kitty and paid for things that way and I think that's a good way to do it.
I've supplied my vehicle on many trips (not necessarily trailering a bike mind you). I've never asked for w/t money. To me that just gets a little nitpicky. While wear and tear on 2600 miles may not be anything to sneeze at, it's not really a big whoop either. If you're hauling a trailer w/2 bikes, you obviously have a vehicle that is equipped to do it so I don't think you're talking any huge dollars here. What, maybe an oil change?
I'm in no way trying to be insulting. But if you're going to start charging depreciation, tire wear, engine wear, etc etc etc, then that sounds kinda cheap.
Although I would be insulted if the guy who isn't supplying the vehicle/equipment said that he was only going to pay 1/3. That's rediculous.