The above was my "quick fix". A better fix for DipCap would be to convince other countries to make trades they wouldn't otherwise - which of course means that there would have to be a change in how various commodities are prioritized.
Let me explain.
Things like gold and plain materials are easy to come by. Your population provides gold, you can make worships to create materials. Those things can run short at times yes, but they're "common goods".
Things like metal, crystals, elementium, horses, etc, those are uncommon to rare commodities. Other civs should be more hesitant to trade uncommon goods for common goods and there should be a higher exchange rate which can then be modified or made up for by spending DipCap.
There should be a law of diminishing returns in there too. The less of an important resource my opponent has, the more he should demand for it. As it stands it's silly that i can use a bit of gold and plain goods to get my AI opponents last 3 horses, or the last of their crystal supply.
I doubt this part of the game will get looked at right now, but I'm hoping at some point when diplomacy gets addressed, trading will too.