Cheers,
upgrade costs seems to be tied to ships cost, but not like being a fracture of it, but a multiple.
So upgrading a ship of the line is much more expensive than upgfrading a sloop, but both upgrades surpass the ships overall cost, and I agree this doesn't seem to be logical.
I am not sure how this is best be solved.
I think the idea behind the immense costs is to make upgraded ships rare and upfgrades at all more valuable. But upgrades being more expensive than entire ships simply doesn't make much sense.
I see two solutions that would solve it without being grossly illogical.
One way may be to simply greatly increase the costs of a ship.
If anything in an age of sails game shall be really expensive, it's the ship itself.
The upgrade costs then can be rduced to a fraction of the ships cost.
Of course, this means taking ships as a way of getting them becomes more important rather than buying them.
Another way would be to make upgrades generally unavailable at shipwrights, but make them sort of blueprints or secrets that have to be found. Could be done in rare books in treasure chests, or something like that. Once you found the recipe for the upgrade, you can do this upgrade at any shipwright. Of course this would require a lot more coding. But it would also move an upgrade from something that anyone can get with sufficiant money to something only few captains know because of their long adventures and experience ... I think I'd like this one more. But again, mcuh more efford to get there for you modders and coders