Actually I've never understood why you couldn't trade cities for money. For example, if you conquer a city you don't want you can give it away but not ask for a payment in exchange. It was not uncommon for kingdoms to accept paying high amounts of money in exchange for a city they considered theirs.
IMO it should at least be possible to buy cities in stable (core) provinces, so that civs can expand realistically without necessarily fighting wars. It's really annoying when your ally conquers a city you want and you can't ask him to exchange it with one of your cities.
It was also a sign of good will in the middle ages to make exchanges along with a peace treaty, which would mean in civ terms that both civs could give something to the other (your ennemy might not want to give you one of his cities to make peace since you're not winning the war that much, but he might think it over if you propose a technology in exchange).
IMO it should at least be possible to buy cities in stable (core) provinces, so that civs can expand realistically without necessarily fighting wars. It's really annoying when your ally conquers a city you want and you can't ask him to exchange it with one of your cities.
It was also a sign of good will in the middle ages to make exchanges along with a peace treaty, which would mean in civ terms that both civs could give something to the other (your ennemy might not want to give you one of his cities to make peace since you're not winning the war that much, but he might think it over if you propose a technology in exchange).