The question is a bit confusing to me. You aways buy traffic, whether it's paid advertising, seo, email blasts, or whatever, in terms of money or time traffic always costs. In the end, like Kay said, it's all about ROI.
Cheap traffic, in my experience, is not that cheap after all. Because 99% of it is absolute junk and simply bounces, you can pretty much adjust the price by 10000% to know the real price. PPC or banner ads don't look that expensive after that.