[alasyr]

I took a look at retrofit number for smaller ships and from the retrofit cost is seems that adding new item in design cost 75% (number that Drake suggested was 70%) and removing item from desingn cost 10% of item value.For retrofit you pay max.
10% of 2/3 value of a new design (it's actually value of current design) and 75% of 1/3 of a new design (difference in resources between designs) resulting in 6.66%+25%=31.66% of a new design which is by 1/3 (33.33%) richer in resources than design it derivates from.So in worst case you save 1.66%.