How about this - instead of working out an economy based on the GDP. Have a "target budget" which is what it would cost if you took the middle cost option every time. (When there's only 2 options use the mid-point. When there's a 4th option use the highest as an extra "very expensive" choice).
Here's the data for the Darwin:
Darwin | | | | |
| Low | Middle | High | Very High |
shields | 7.9 | 9.9 | 12.0 | |
Primary Hull | 32 | 36 | 54 | |
Deflector | 0 | 1 | 2 | |
Warp Core | 0 | 2 | 4 | |
Theoretical cost | 27.9 | 36.9 | 60 | |
Actual Cost so far | 40 | | | |
Engines | 5 | 7.5 | 10 | |
Phasers | 8 | 10 | 12 | |
Theoretical cost | 40.9 | 54.4 | 82 | |
Actual Cost so far | 53 | | | |
Torpedoes | 4.5 | 9 | 12 | 16.5 |
Theoretical final cost | 45.4 | 63.4 | 94 | 98.5 |
Actual Final cost | 69.5 | | | |
Target Budget: | 109.6% | | | |
As you can see, The actual cost of 69.5 is only 9.6% over budget of the theoretical 63.4.
Here's the data for the Federation so far:
Federation | | | |
| Low | Middle | High |
Shields | 11.4 | 14.05 | 16.7 |
Primary Hull | 9 | 15 | 22.5 |
Hull Slope | 5 | 11 | 15 |
Warp Core | 4 | 8 | 17 |
Engineering Hull | 16 | 16 | 16 |
Nacelles | 11.5 | 17.5 | 23.5 |
Engines | 8 | 12 | 16 |
Theoretical cost | 64.9 | 93.55 | 126.7 |
Actual Cost so far | 99 | | |
Target Budget so far: | 105.8% | | |
Phasers | 18 | 30 | 50 |
Theoretical cost | 82.9 | 123.6 | 176.7 |
Theoretical Actual Costs | 117 | 129 | 149 |
Theoretical Target Budget: | 104.4% | | |
So far, we are at 99/93.6 = 5.8% over budget. If we went for the 30 cost phasers it would be 129.6/123.6 = 4.8% over and the 50 cost phasers would be 149/129 = 15.5% over.
The cost rating could then be based on what percentage of the target budget the ships cost is:
S+: <80%
S: 80% - 90%
A: 90% - 100%
B: 100% - 110%
C: 110% - 120%
D: 120% - 130%
E: 130% - 140%
F: >140%
So the 30 cost would make it B class, while the 50 cost would make it C class. And there are still the torpedoes to be fitted.