I think you should either make the cost exponential too, which would involve MATHS, or we simply agree that "the separation out of your guns is just for funs". And you just record how many points worth you spent on each weapon system next to that weapon system, and that's its "power level".

So, for that battery of 10 20 CM railgun turrets, you record that "40" point value as the power of that weapon system.

If you want a little differentiation between lots of little guns and a few big guns, a way would be to say that lots of little hits lower shields more effectively, but big shots pierce armor better. (But not as much better as actual math would imply. Because a 100 pt 5M railgun would impart the same kinetic energy as 2500 10 CM railguns.) Or maybe hand wave it with, "little guns reload/fire faster".