Asking us to pay for a meal; Is this bad etiquette?

I'm living in student accommodation in a flat. On our floor there are three flats and we all every-so-often cook for each other. My flat and another one have never asked for money for this. However, the cook in the flat next door has asked us to pay for the 2 out of 3 times he has cooked. The portions are decent sized. but as there are so many of us, i'm pretty sure the amount he asks of us exceeds what he actually paid for the ingredients. Furthermore, on one occasion, he asked us all to clean up his kitchen for him so he had room to cook. we had been at his flat the night before and used gles to drink out of, but he expected us to clean more than that and he didn't even help! I feel a bit put out because he is the one offering to cook for us, yet expects payment and I accept his offer out of politeness more than anything else. Am I being too sensitive? and If not, is there a polite way to decline without being rude, when all my other flatmates go over?