How are you calculating time cost?

I can cook crockpot dinners with 15 minutes total time (prep, check time, washing/cleanup), even though it takes hours to cook. This will prepare ~5 meals for both myself and my girlfriend that cost maybe 1.5 mins each to reheat later. At a total time of 30 mins input for 10 meals worth 5 dollars of saving each, that's a cost of $100/hr.

That's a ridiculously efficient use of time. Even if you're a third as efficient and take 45 minutes prep (the 15 mins to reheat should be constant), you're still running $50/hr, which is still excellent. The initial learning cost for crockpot cooking is very low as well.

Do you calculate this as 4 hours spent (fire time) or 30 minutes (your time spent)?

You can do similar things with stews, rice+stock dishes (gumbo/jambalaya/etouffee/paella), sous vide, and roasts/loafs. These are the easiest. Once you get better at cooking, you can expand and cook other dishes a similar way. As you get better, you spend a smaller and smaller fraction of time actually attending to fire.