This means they expect money. How long have you lived here, people say that just to appear humble. It's like when you're entertaining people and they say'here, let me pay for it'. This means you should pay for it, otherwise you'll just ruin the relationship. I've even spoke to elderly cab drivers who refused fares, doesn't mean you don't pay for it. It's not even like they want to have a big dinner, it's just that they are forced to do it, so help them out a bit. When it comes to your turn (if it does) and you are Chinese you know it will happen to you too, and you will surely want them to help you out.