in theory you could use rarity and cost for this but is only temporary solution
you might try different approach and to introduce each era as faction
e.g.
Federation-TOS
Federation-Enterprise
Federation-TNG
Federation-DS9
Federation-Voyager
this way you may keep each easier to balance
plus newer variant of Federation can still use older vehicles in theirs 'sets'
generally there isn't anything as introduction date of shiphull value (to determine it's age)
that's something to ask developer of SS to add or create whole mod for it