I have a question. What's wrong with coming out with a paid app for, say $2.99 and then pay 51% to let's say TapJoy? If they don't drive someone to buy your app, you don't pay. If they do, then you get 49% - 30% = 19% of the app. Isn't this essentially risk free and improves your sales numbers?
CPA is certainly lower risk than others. The only minor issue that I can think of is that you (as an advertiser) essentially push the risk to the publisher. So, the big publishers won't be interested in this model. They don't control your conversion funnel, after all.
I'm saying this as someone who used Offerpal (which is now called Tapjoy) as a publisher.
There are absolutely legit CPA offers out there, and sometimes it's a good deal. I don't know tapjoy personally, but I do some CPA deals online for brands you're familiar with. If the customer isn't known/predicable they get the absolute bottom of the barrel ad inventory until we know whether it converts at all. If it doesn't convert at all, we cut you loose. If it does, I squeeze every penny I can out of you, and hopefully everyone's happy.
If you can, it's worth investing time/money to learn the market yourself.
