If I'm trying to save money and I order only a $5 appetizer and you order a $15 entree and $5 dessert, is it really fair for us to split the bill in half?

(I'm not talking about dining with friends and family here, but if you were eating with a co-worker because of work, like you had to share a lunch because you were at a conference together. This happened to me the other day. I didn't want to be petty over money so I agreed when my co-wroker said "it would be easier to split the bill," but now that I think about it, I should have said something, maybe as a joke or something. But I think it was pretty dumb of her to suggest that we split when she obviously knew she had spent more money.) Anyways, just a thought.