Hi theinsightco,
I'm not sure if you are talking about getting income from PPC or paying a PPC for advertising, I guess it is the first.
I find Google (ASI) give a good return compared to trafficz.com, but they have a big disadvantage, at the moment, that there is no breakdown of the revenue or visitors per domain, so very difficult (read impossible) to assess how different types of do there compared to elsewhere. Hopefully this will change.
Also, they say they require the exact domain name (plus www) to be used as the "keyword". For residual traffic domain names this is not advantageous in most cases for maximising income.