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 spell or through having Marid 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.

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.

In PBH you don't have anything similar to what you are looking for, but in core books, DMG has some data which would be helpful for your DM. If you want to use only PBH @Upper_Case answer is amazing.

As I DM I would assume that this project will require an regular similar amount of work as building a keep or small castle, so according to DMG p. 128, it would cost 50k gp & take 400 days.

The bigger problem is the making submarine to support the travel and living undersea.

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

Taking this into account 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 to make submarine useful.

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