I'm going to shoot for a one-time purchase deal. The price will likely go up as the game itself gets longer, since it's a LOT of work putting something like this together, and I plan on this thing being upwards of 24+ chapters, all at the very least as long as chapter 1, if not longer. (holy run-on sentences, Batman!) Look at the cost set up as a "lock in your rate now" type thing.
It's the most honest way of going about it I could think of.