What about Trade Routes?
Sidebar: Trade Routes. Specifically, Internal Trade Routes.
Real Trade is about moving stuff from one place to another. You grow some, er, corn?, and then you stick it on a truck train ship and you bring it to me. I send something back, like money or Kardashians. But not in Civ. Instead, Trade Routes magically just create the Yields they deliver, whether it's hammers, gold, food whatever. The local city doesn’t trade away any corn or hammers or science - the trade route just magically makes more.
Totally unrealistic. And I'm totally okay with it. Because while it's unrealistic, it is easy to understand and play with. And it's also actually not totally unrealistic. Well, not totally unrealistic if you’re willing to squint. Really, Trade does sort of make something from nothing in terms of economics. If you start trading food, your food might go down in the short term, but in the long term you'll generate more food because of increased demand leading to increased supply (partly because you’ll get more corn or whatever out of the same land / factories etc). I think the Trade Routes sort of get at that, which is why I think the creating yields from nothing is at least justifiable, and like I said doing it this way is probably easier to balance.