C2C is in urgent need of a complete review on the monetary value of units. The current values are game balance breaking.
IdioticUlt1mara is doing a stream on twitch (*) and the below exploits almost break the deity/nightmare difficulty.
(*)
https://www.twitch.tv/idioticult1mara
1) The AI overvalues worker units (and possibly other units) in the trade screen waaay to much. Which means they trade you a tech for 1-3 worker units each after you reach writing. Mass produce worker units and you can buy tons of tech for a small fraction of the time it takes to research them yourself using the same amount of hammers to build research.
2) Disbanding a military unit within your own city gives you a sizeable chunk of cash. More important however, if you have Size Matters on, and you split the unit into 9 smaller units, each of these smaller units sell for the same amount of cash as the original unit. Thus building military units, splitting them and disbanding all of them gives an extreme amount of money.
Exploit number (1) allows the player to catch up very fast in tech, which largely negates the research advantage the AI gets on Nightmare.
Exploit number (2) allows IdioticUlt1mara to spam cities in Nightmare mode and get a massive negative upkeep cost (still negative after setting 100% cash + all cities building wealth) but he is able to keep financially afloat by building 2-3 units every turn, split them and sell them, with 100% research and no cities building wealth.
Exploit number (2) should be game breaking in multiplayer too.