I would like to know how much it would cost to factor a large integer. The cost can be given computer operations, time to process it or even monetary value. I know there are people that factored 200 bit RSA keys, but Wikipedia doesn't say how long it would take in modern CPUs.
-- Background --
I am designing a virtual roulette game where the ball is spun (the number is chosen) before the player bets. As a proof of that but I show the player the product of two primes, one of them is used as the random number seed.
I would like to know what is the minimum suitable size for this number such as the cost of factoring it will be greater than the maximum prize I offer.