From what I've gathered, free apps are downloaded multitudes more than paid apps. Having a paid app out there myself, without a massive advertising budget, sales are not good. It's an excellent app, just that there is no way for anyone to know it's good unless they know it even exists. A free app, even with ads, will get far more downloads and at least get your name out there to build on. Using the ad networks is nice if you have enough users to generate some income, and with a huge user base can make way more money than the app could if it were a paid app. The trick is getting it downloaded in the first place. Advertising your own apps is an excellent idea, because then a successful free app (the app that has a greater chance of getting downloaded a LOT more) can help to feed the paid apps some downloads.