Well...I don't think "cheap" is the word that you want to sum up the traffic, but there are plenty of inexpensive methods to getting high quality traffic.
The content suggestions above are good, but you have to want to develop content. You can always find a targeted newsletter and buy advertising. Link buys on other high traffic sites work as well. Myspace is a huge network, and if you have a relevant product/service you can recruit people with a huge friend list to promote for you.
As for SEM. You can definitely buy high volume $0.01 traffic, but it will most likely come from content matches and therefore you can expect the actual conversion to be quite a bit lower.
You should always build out your site for search engines and adding great content helps, even if it isn't immediate you're going to be happy 6 month - 1 year later when it starts bringing in organic traffic.
Of course there are always affiliates...but that doesn't fall in the cheap category when you look at the overhead of running a successful campaign...hmmm, maybe changing your focus to "what works best" and finding out how to maintain that is a better idea
