The miner pays real money to his electricity supplier to run the mining hardware. But aside from that, there is no overt cost (in the BitCoin world) of mining coins. The benefit is that you create bitcoins, but as indicated, you create them out of real money that paid for real electricity made from real uranium atoms.
There's also a sort of opportunity cost, in that if miner A creates a coin, then miner B cannot create that coin. It really is a zero-sum game, and if I've understood the operation correctly, there is a finite, fixed supply of possible bitcoins, of which some non-zero number have been permanently lost for a variety of reasons.
EDIT: footnote: *your* electricity might not be made from uranium atoms, but 85% of mine is.
