The cost is very low through services like Mechanical Turk. Typically you'd ask them to complete a survey on usability of your site -- you get some useful feedback from them, and sometimes they return as users themselves.
But do you get paying users from a site like that? If I'm anonymously filling out surveys for $4 per hour I'm probably not in the target audience for your $10 per month SaaS service or a particularly prolific purchaser via affiliate links.
Does anyone have any actual success stories in that area?
