buying traffic is in no way to be compared to PPC or SE traffic - but you get volumes of traffic at cheap prices - sso the way to compare is by $$ and not per visit.
So, if you buy traffic at say $2CPM (1,000 hits for $2) then these 1,000 hits/visits should be compared to the result of 2 $1 clicks in for example adwords.
Also - it depends a lot on what you´re marketing. Buying traffic works best for "general audience products" - stuff like "making money online" or "online poker" is examples of subjects that attracts almost anyones attention - thus you don´t need the keyword target of PPC or SE traffic.
I usually check out clickbank or other affiliate sites for new products to market - then I buy expired domains traffic, 5 or maybe 10k (you get it as low as $20-$40 from quaity sources like buyexireddomaintraffic or redirectedtraffic) - that way I can get lots of traffic in a short time and that gives me more accurate stats to evaluate if this product is worth marketing "big time" or if I should just drop it and move on
/Peter




