Don't purchase cheap traffic if your looking to get conversions from it. The services that provide cheap traffic do so in a few ways, but basically they have some click thru program with has some kind of incentive for people to visit pages.
The more pages they visit the more they will get. What this means is that the paid visitors don't even look at your page, it doesn't matter how great your landing page is or what fantastic offers you have on it.
So you might ask what the point of buying cheap traffic is then, if it isn't going to convert. Is it just some kind of scam? No, it isn't a scam, you just need to understand the purpose of buying traffic that isn't going to convert. Basically you are trying to water down your conversion rate, or hype your traffic stats.
Lets say you're promoting a cpa program, and you are using some bh techniques to get people to the page. If too many people are completing the offers, it will look suspicious to the cpa. So to make your conversion rate seem lower you can purchase a traffic package, have the traffic delivered at a rate a little higher than the traffic you're already sending to the cpa, and it makes your conversion rate seem more reasonable.
Or let's say you want to make some websites and flip them. Well if you just have a website with no traffic, then you aren't going to get a very good price selling it. So if you purchase a traffic package and have the traffic delivered at a reasonable rate, it will make the site look like it is worth more money. Keep in mind that you'll be taking advantage of people just starting out. Anyone that knows anything about running a website will be able to see the difference between purchased traffic, and natural traffic.
So bottom line, if you are needing real traffic to your site that might look at your offer and make a purchase, you aren't going to be able to just throw a few dollars at it and magically be making money.
It is possible to purchase traffic that will convert. Things like Adwords and other PPM or PPC are paid traffic services. There are other ways to pay for quality paid traffic as well. But anykind of traffic like that is going to be a lot more expensive. Otherwise why would we all be working so hard to get good rankings in google, or get reall backliinks on front page of digg and things like that.
If all it took was to pay a few bucks for traffic, everyone would just be doing that.