[SumZero]

I would think he should get around 25%. Basically, you could imagine it like 2 different tournaments. In the first you backed him and he played and you split the prize pool, say, 50/50. In the second he's backing you with his half of the winnings from the first tournament. So your entry fee is essentially half your money and half his money. And now for his half of the money, since you are playing, you get 50/50 split of that 50% which is what gives him the 25%. Even if you thought the backer should get more like 2/3 of the prize money that still works out to >22% for him.