I’m at a work conference right now, and during the less gripping moments, I open a notebook and discreetly map out the final complex disposition of factions in Game 5 of Choice of Rebels. On the plane to get here, I finished one book on French peasant heretics and got through another on Wat Tyler’s revolt, taking notes throughout on ideas I wanted to borrow/adapt for Rebels. I’ve got WhatsApp conversations going with friends about the Abhumans and Halassurqs that are helping me flesh out those rival civilizations.
All these things bring me great intrinsic enjoyment. Otherwise there’s no way I’d do them…it would all be a bit over the top for an economic enterprise! If I measured the hours I’ve put into Rebels, especially counting those “ideation” ones, there’s no way I’d make minimum wage.
But because I’m writing mainly for the joy of it–of having a creative side project that produces the kind of story/game I’ve always enjoyed reading–that doesn’t trouble me. The earnings are a terrific side bonus. So is the opportunity to read and think a little deeper about the relevant history, philosophy, and politics.
Personally, as someone aware of the research into the fragility of intrinsic motivation, I’m very keen to keep extrinsic motivators mentally at arm’s length. I’m not taking any advances for Rebels Game 2. I won’t be counting words or hours for Game 2, any more than I did for Game 1. I’m trying to reduce the frequency with which I check how Game 1 is selling. 
To keep myself motivated, I focus on the intrinsic rewards. I get my ROI every time I re-read Rebels, or have a fun discussion about it on the forum or with friends.
I don’t think there’s anything wrong with extrinsic motivation. Someone who starts a project with the clear intent of putting food on the table and writes as efficiently as possible towards that goal can produce a fantastic game. But it wouldn’t get me to the finish line with Rebels.
I would offer one note of caution to anyone who’s already started writing for the joy of it, and who might be tempted by this thread to start counting hours and calculating a cash ROI. A sizeable body of psychology research into human motivation suggests that once you start focusing more on extrinsic motivators, you risk killing off the intrinsic motivation that got you started–and then you might never finish. Not even because you discover the ROI is terrible, but because once you start calculating opportunity costs, there are a lot of other things out there to do, and it erodes the joy of creation that was the real reason you started.