decrease credit cost by 25-45% (50%?, unlikely, mine spamming would probably ensue)
decrease other costs by 30-40%
decrease construction time for TEC's mines. The other 2 races can deploy a field of 10 in about 0.5 seconds. The TEC take around 10-30 seconds (depending on the constructor count, constructor location before construction begins, etc. etc.)
With all the mines, I think an increase in damage would be good. 40-50% increase. Additionally, perhaps having the Vasari's mines be able to put a max of 30% of their damage through shielding and directly hit the hulls of enemy ships.
With the TEC/Vasari mines, a radius increase for the damage is a great idea. I don't theink the Advent need this, considering that their mines move/home in on targets.
Gravity Mines- wide area effect, and have the ships either a. completely immobile or b. moving extremely slowly