Unit Upgrade cost Broken?

I would rate it as "too inexpensive", not sure about the exploitative tag yet. (In Marathon, it cost me 250g to upgrade my warriors to swordsmen.)

The sweet spot would probably be a 5-10% savings or so if you manage to time everything properly with creating the warriors right before they get can get upgraded. Not game-breaking, but a small reward for smart play.

 

So your problem here is that a strategy game is rewarding a player for planning ahead and executing a plan instead of just throwing money at a problem at the last minute? Yeesh...

 

Then there's the issue that you need to have all that iron hooked up (or traded for) so you can support your warrior -> swordsmen rush buy.