Basically all of the open mobile platforms are economies with a very long tail -- In other words, a relative handful of the most popular apps make "real money" and revenues fall off exponentially from there. Without having actual figures to back it up, my impression based on experiences and studies I've read are that 98% of developers are making "pizza money" at best -- $100 US a month, or less. There are millions of developers and millions of apps -- hundreds or thousands of new apps each day.
Even good, high-quality games fail to find a market every day in these economies, for whatever reason. They maybe lack marketting, or launch against other software that grabbed all the mindshare the morning they launched. Less scrupulous publishers hire shadowy firms to inflate their initial download numbers so that they can raise into the public's conciousness, in hopes of cracking the top-10 list where the real money is.
All that is not to say that success is impossible, but it certainly is improbable. There are still success stories, but the biggest among them tend to be games that have 'gone viral' like flappy birds -- the polish and appeal of such games play a part in starting the viral reaction, but its not really something you can design to influence. Flappy Birds could have ended up making no money at all, had the winds simply been blowing a different direction that day. Frankly, I would *plan* on not making any money to start, and take your first games as a learning experience. You might get lucky, but more likely you'll establish a small but growing fanbase who will have an increasingly large back-catalog of your games to buy and recommend to their friends. Over time this can grow into a sustainable business, but under almost no circumstances today are sustainable businesses springing forth from a single game launch.
Also, a 'box price' (where you buy the game up-front for a set price) is a complete non-starter on mobile in general and on android specifically. The ease with which (cracked) games can be side-loaded on android means there's a vibrant android warez community who simply never pay for the software they use, and even among the more honest android users, the ecosystem provides 10s or hundreds of free alternatives in nearly any category, no matter how niche. Instant, meaningful success on these platforms has almost nothing to do with the quality of your product -- although a quality product is a minimum barrier to entry -- and everything to do with factors you can't control, like getting some good organic publicity or serendipitously stumbling onto a top-10 list on the back of viral forces.
The easiest way (most easily grafted into any kind of game with minimum effort) is to build your game to support adds and have it be completely free, then, as an in-app-purchase, offer an item that disables adds and sell it for $1. Crackers will even remove the adds, probably, but usually people will just take the official free version with adds rather than risk whatever other malware a cracked copy might contain. So, you'll get a trickle of revenue from anyone who plays your game, and you'll have the opportunity to make sales to those who like your game enough to want to skip the interruptions. Another idea is to simply give away the first few levels or hours of content entirely free with no adds, and then sell the remaining content via in-app purchases -- that's essentially how iD software established their business back in the day with Wolfenstien3D and Doom using the then-common shareware business model; of course, the shareware markets of the day were much less crowded, your only competition were the other floppy-disks sharing the wall at your local radio shack or mom-and-pop computer store.