I've used Vouchers for Adwords, and I didn't find much traffic coming from that. And the cheap traffic you are talking about is probably 1000 visitors for like 5 dollars or so? That's non-quality traffic coming from pop ups, or pop unders, or maybe surf sites. Not worth it.
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If you're talking about the type of websites that offer "1,000 unique visitors" for $10, then you won't receive any good traffic. These websites are most likely scams. They use automated bots to bring traffic to your websites.