I'd rather go with a rental model, where I get a certain time period (a month?) to play the game, after which it goes away. There are very few games that I play more than once, since time is limited and there are so many games to experience. Usually, after beating a game on Steam, I immediately delete it to clear the SSD space.
Paying 50c per play could work for demoing a game, as long as the save was kept and whatever money was spent could be used as a credit toward purchasing the game.