I like those math questions captcha. Fun and I doubt a bot or even a real persona spammer will waste time on this. Make the question appear on an image instead of text and the bot will also have to do OCR on top of being wolframalpha to defeat your captcha.
TBH, I had decent results with the most trivial "3+4=" style CAPTCHAs. I know some sites where there's even only a single, hard-coded question/answer.
In the world where it's not about scaling to infinity, you're not getting targeted attacks, you're getting stupid bots that pummel every site thinking it's a WordPress install from 2007 and anything different is enough to scuttle it.
By the time your service is popular enough to justify building a focused bot or tossing low-wage workers at it, hopefully you have the revenue in to finance something more sophisticated.
