I have held games where I not only DMed (minimum 10 hours of prep time.....I'm THOROUGH), provided the venue, provided soft drinks, and cleaned up the mess on my own, yet was still asked to help chip in for pizza.
I love the game. I love the preparation. I don't mind hosting and I don't mind the aftermath, but sheesh....it seems like good form to chip in for something for the DM. I would NEVER ask for anything nor would I expect anything, but it just seems like good manners to offer from time to time.. I can't even imagine asking someone to pay, that just seems wrong. But the occasional, "Hey DM, put down your wallet. We got the bill!" would be appreciated.
Oh well. I was just raised different I guess.