There are 2 forms of cost-PR ratios.
they are initial cost and upkeep costs.
im assuming you mean upkeep costs but i will post both cos i`m helpful. =D
Initial cost/PR = Scorpion Starbase
Upkeep/PR = Scorpion Starbase
Having the largest PR is not always the best however. once you have learbned to stack. (balance your fleets with a variety of ships) you will win more battles, also try to include at least 1 neutral ship when your building your stacks as it halps to throw off enemies if they think your using all your race ships and no neutral ones.
also a few tips for terran.
fodder herc works well also
fodder thor too. =P
have fun