You're not going to make any significant profit mining bitcoin on a desktop CPU, or any "normal" CPU for that matter. If we assume 5 seconds of CPU time per co

You're not going to make any significant profit mining bitcoin on a desktop CPU, or any "normal" CPU for that matter. If we assume 5 seconds of CPU time per comment, that's ~17k per day or ~500k per month. The first captcha solving service I found sells 100k solved captchas for $139, so that's about $700 for 500k. As a spammer, I could probably post 5 to 10 times more comments for the same amount of money using your system. This is obviously a very rough estimate, but it should get my point across.