This came up yesterday with a student who wanted to have lunch at a restaurant at a nearby airport in between lessons. I make it a point to bring my lunch to work everyday so I don't waste money - I'm on an extremely tight budget (I'm a CFI, so duh). It's not really OK with me that I would have to buy my lunch just because my student wants to hit an airport restaurant, so I let him know that if he wanted to do lunch, he'd be buying mine. It came up quickly and I wish I'd had more time to think about it - I'm the kind of person who likes to have everything understood and out in the open, so I risked an awkward moment to be sure we had an understanding. He was fine about it, but I feel kind of cheap. I wish my budget wasn't so tight so I could just say "sure man, let's go!" Chances are he would pay for it anyway.