I would assume that this project will be as grand as building a keep or small castle, so according to DMG p. 128, it would cost 50k gp & take 400 days. 

You would need to solve the problem of moving through water. Maybe thanks to artificer created engine, through a magical item with [Control Water][1] spell or through having [Marid][2] for your services.

Assuming that you move through water, you still need air. So you would need to magically enhance your submarine to do it as well for its guests. There aren't any spell which creates air, but you can use the [discussion about water breathing here][3].

I would put a price of a legendary magic item, DMG p. 129, so require 500k gp and require 20k days of work on it.

In total 550k gp & 20,4k days.


  [1]: https://roll20.net/compendium/dnd5e/Control%20Water#content
  [2]: https://forgottenrealms.fandom.com/wiki/Marid
  [3]: https://rpg.stackexchange.com/questions/86701/is-there-a-spell-that-lets-you-breathe-in-an-airless-void