ok, I was messing around with a test server, and this is what I got.
EDITED:
capacity = (power*param)
recharge = (param/16)
cost = (capacity*200)(All recharge values are the floor value. ie 12.5 = 12)
150/315/500
CAPACITY/RECHARGE/REPAIR RATE
shield1: 400 12 33 per/s 3 Blasters 2 Rails 1 Torp
shield2: 900 18 50 per/s 6 Blasters 3 Rails 2 Torps
shield3: 1600 25 64 per/s 11 Blasters 6 Rails 4 Torps
shield4: 2500 31 80 per/s 17 Blasters 8 Rails 5 Torps
shield5: 4800 50 96 per/s 32 Blasters 16 Rails 10 Torps
shield6: 7200 75 96 per/s 48 Blasters 23 Rails 15 Torps
shield7: 12600 112 112 per/s 84 Blasters 40 Rails 26 Torps
shield8: 14400 75 192 per/s 96 Blasters 46 Rails 29 Torps
4*800 = [3200 capacity] [50 second recharge] [640,000 credits]
8*400 = [3200 capacity [25 second recharge] [640,000 credits]
How does this whole shield thingy work?
EDIT1: also, it might be a good idea to impliment some way of adding cost according to recharge too because I could make a
-100 power shield*10 and have an uber recharging shield thats cheap as dirt... i think... lemme check to be sure

EDIT2:
ok, I made a -1000 power shield*10
nothing short of plowing directly into 5 mines or setting off 4 fatman bombs centered directly over you will kill you... the recharge takes less than a second, and has 10,000 storage... all for only 2mil
I ran up to 4 enemy starbases and was merely pushed back by there firepower, even sitting in the middle of the sun, nothin.
On the other hand, a -10 power shield*1000 takes 62 seconds to recharge.