Depending on the network, you may also need to pay setup costs. This covers the cost of integrating you to the network and testing tracking etc. Start-up fees can range from nothing to over $5,000. I have been asked for $30,000 before and, obviously, I never started with that network. This cost is often negotiable when you discuss contracts with the network, but it is worth noting when considering the start-up costs.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.