Anyways: if you make it expensive to pass a captcha, you will scare off regular users, if it is dead cheap, bots would just pay.
What might work is a (web-)miner and a getwork source. Mine one or more difficulty 1 shares and you can enter.
There is a direct relationship between shares & coins.
1 diff 1 share = 1 BTC / (current difficulty)
Even 1000 diff 1 shares <= 0.07 bitcents.
So there is no real advantage to using shares instead of coins. Coins are simply "purchased shares".
A modest fee would make bypass for the purposes of spamming prohibitively expensive.
Current captcha bypass solutions are on the order of 25 bitcents per 1K solutions.
That is 0.04 BTC per capthca bypass. Allowing legit users to bypass the captcha for 1 bitcent would increase spammer's cost by 25x while not costing user much.
I would love a captcha bypass system for 1 cent per capthca. Eventually as more users use the "humancent" system you could make the captcha more timeconsuming driving up spammer's cost and eventually move to a "no-captcha" system where user must pay a micro tx to continue.