"Fair", according to the rules of rationality? An arbitrarily large amount of money.
"Rationality", as understood by economists, says exactly nothing about this. You may be rational and not be willing to pay more even a penny for this though if you like money, you should be willing to pay at least $1 (but even that's above and beyond rationality - this is just first order stochastic dominance argument). The expected monetary value of this lottery is, of course, infinity, but this has nothing - exactly, nothing - to do w/ rationality. Most rational people, in my experience, would not pay more than $10 for this. Again, as economists understand it, there is nothing "irrational" about that.
Just in case: all rationality means (for economists) is that an individual is maximizing a complete and transitive preference relation over the set of lotteries. A violation of rationality would be, after paying 10 dollars for this lottery, he exchanges 10 dollars for 75 apples and then exchanges the 75 apples for the initial lottery. There is nothing, really nothing, about maximizing expected monetary values, in the definition of rationality.
