Sunk cost fallacy. You aren't getting that $7.50 back. The question should actually be "do I want to, for no reason at all, feel like shit for the rest of the evening?"

(This assumes you aren't deriving any pleasure or utility from eating that last 25%. If you are, then that's one side of the cost/benefit analysis. But the initial cost of the food is irrelevant not matter what.)

I can ‘spend’ the money on the remaining 25% of food, or on not feeling shit (e.g. I didn’t ‘get’ anything for that money).

Yeah. I suppose sunk cost fallacy here would be "I should eat it all to avoid wasting money." I guess you're sort of actually psychologizing OUT of that. It's still a bit fallacious, since the correct cost of the decision to not eat is actually $0, not $7.50, but I guess lying to yourself about that could be a useful mental trick.

> It’s much harder when the food is great though.

Indeed. I doubt anyone ever got fat by eating food they didn't like.

The issue becomes all the worse when food is designed to make you want to keep on eating. Bonus points for it giving you that sort of hunger-like feeling 2 hours after your meal.