As long as a shared lead is delivered in real time, uses key-word search engines to generate the leads, (not banner ad or incentive ads) and has a decent return policy you're after the cheapest lead you can get your hands on.
Watch sites touting their leads are "better" or "more qualified" then other sites. No one has a more qualified leads since clients simply stumble onto a variety of sites and plug in their info.
I once fell for this - paid $15 a pop for leads touted as "high quality." I got destroyed.
This is a numbers game and you need a lot of leads. I'd never pay more than $8 a pop and you're looking for less than that.