Fair by design
Most zombie games on a phone are free to download and then charge you to keep playing: an energy bar that empties, a timer you can skip for coins, a crate that drops better guns if you pay, and an ad between every two hordes. Town of the Undead does none of that. You buy it once, and that is the whole game.
Not between hordes, not for a bonus, not on the loading screen. The game never connects to the internet while you play, so there is nothing to serve an ad from. That is also why it plays offline and collects no data.
30 levels of story, seven places to drive to, two dozen neighbors to rescue, 200 achievements, a Tool Room for crafting your own weapons, and New Game+ when the story is done. Updates are free; the game has had a new version most weeks since launch (see updates).
Town of the Undead is a cozy game: ten minutes of calm, then a horde that's over in a minute. A cozy game cannot keep interrupting you for money. The one-price model is what makes the short, calm sessions possible.