But old technologies are also most costly to repair in terms of utilising older components that are no longer in standard use so are no longer produced in high frequency.
While that is true most often high maintenance cost for old equipment is the fact that it is old and many parts of the construction can't be replaced and even if you put in some new parts things don't work like it used to... but on the other hand you have very experienced crew who knows the equipment very well and so are very good at fixing and reapiring it... but over time... yes old ships should become more and more expensive to maintain.
But old technologies are tried and often cheap to produce and maintain and as long as they are still in use and you create new components they should be cheaper and cheaper to maintain.
Not sure how viable it would be, but the game could track how many components you have of one type and then use that as one of the parameters to maintenance cost... the more you have of the same component in a fleet the cheaper it should be to both produce and maintain... So... new components would be both expensive to produce and maintain at the beginning but become less so as you get more of them. If you have specific components that you have very few of... say a very large active sensor system then that component will be expensive throughout the entire life cycle... which seems realistic.
You then also could get the same effect as in reality... small component generally are more expensive to produce and maintain due to being more complex for its size compared with a larger component, in terms of price per ton that is. But, you then would see smaller components getting less and less expensive both to produce and maintain as you start mass produce them for yor entire fighters fleet, say a particular fighter engine that you will have like 200 of. But 2000t cruiser engines might be cheaper per ton to build at start but will remain quite expensive as you might just build like 20 of them or so.
It would be very difficult to know how expensive your ships are and will become in the future as it relies entirely on how many components you will build of the types you have in them. A ship can be quite inexpensive after a while when you have a large amount of them.
It would also show they more realistic approach to jack of all trades ship versus more specialised ships... it would be more efficient over time to build all ships with the same sensor suite even if they are not optimal for every ship from an economical perspective. These are real life decisions militarise have to wrestle all the time. Good enough versus the best but very expensive... there are rarely a best answer to that question.