It still comes down to 'am I willing to invest enough time to get what I want or find out it's more expensive than I'll pay.' Playing efficiently demands it, and the only thing stopping efficient trades is frustration factor at a clunky interface. Being able to accurately judge the value of the items we're putting on the table is a convenience and time saving factor. Instead of keeping notes on trades I've made in the past to try to divine the values of different items at different times, with it always coming back to GPT, I get an exact representation in-game.
Endless Legend has a good example of such a trade system at work. You can get a general idea of how much this trading partner values what you are offering and what you are asking for, based on whether the balance bar falls in your favor or theirs. There is even a diplomatic opinion boost for giving them trades in their favor, and they'll never agree to a trade that isn't at least even according to them. It is a much easier to use and more enjoyable trading system than the 'guess, and guess, and guess again' we get with Civ V.