I don't think a chicken is a whole meal. There's surely going to be other things served with the chicken and as someone else mentioned, I'd rather be asked to pick up a chook (which can be done on the way there) than have to prepare a salad. And by the time you buy all the bits and pieces to make a salad, you're looking at the same cost anyway.

I do get your frustration at them not contributing when the situation is reversed, but I think you've put yourself in this situation. In future don't offer if you're unhappy it's never reciprocated. Or flat out ask them to bring something next time, don't wait for them to ask (or not ask as the case may be).