I think in addition to upgrading unts, you should be able to spend money to update the mine.  The mine should have atleast 2 levels that improve income in a significant way.  If I play a game less thatn ten minutes, it shouldn't make a juge difference but in any game longer than 10 minutes, a level 3 mine should make a big difference in your income.  For example:

Level 1 (base): = 250/min (approimately what it is at now)

Level 2 = 375/min (costs 1250 to upgrade)

Level 3 = 500/min (costs 1000 to ugrade)

Level 4 = 750/min (costs 750 to upgrade)

Realize that this is for each mine and there are currently 2 mines.

The levels each ramp gold production and decrease in costs.  The decrease in gold cost is because the money invested in higher levels require investment in the previous levels and thus for level 4 for both mines you invest 6000 dollars.  In order for this to be useful,  the game will need to last for atleast 8 minutes past this upgrade if done all at once.  The 6000 dollars lost that can't be spent on equipment since you spent it on the mines.  This requires a late boom.  I think this adds a different aspect to the game that may be very interesting, and useful for generals in particular.

In addition, mine levels should have short cool descriptions:

Level 1:  A basic gold(?) mine with a single shaft.

Level 2:  Add an automated ore processing unit to increase purity.

Level 3:  Add mechanical core driller equipment to engineers to increase yield.

Level 4:  Add refinery and smelter to further increase yield and purity.

Let me know your thoughts.