Yes, sorry retrofit is 20% more. But you do not pay 10% of the cost of the old design. You only pay 10% for components that you remove. If you design a retrofit series that only components are added, which is easy usually, there is no extra cost. So the cost to construct a retro-ship from a 50% hull is 20% of 50% = 10% more. Still not worth mentioning.
Again, you do NOT have extra maintenence, you have LESS. You have to train your ships anyway, and you pay more maintenance if you train them fully equipped rather than train them as a mere shell.
Yes, you pay more as you can build more. You pay nearly double for more than double the ship production. If you do not have that much resources, ok - still it may be better to use retroseries with half the shipyards and idle the other half. Then you either have too many shipyards, a too high racial shipyard rate, or not invested enough in resource mining.
Not using a resource like shipyard capacity is a waste, and game strategy should be adjusted so it does not happen. Wasting capacity with drawn-out buildings of complete ship designs may give you the illusion not to waste capacity, but you still are at a disadvantage not using your full potential even if you try to hide it from yourself
