How much would U-boat building cost?
 

I'm wondering how much would cost to build or to buy a small sub. (without torpedos and torpedo holes)

Depends on how big you want it, what features you want, when you want it and how much you have to bribe some offical to get a full load of torpedo's and deck gun rounds!:up:

I said a small sub for 5-people crew, with 2 torp tubes (one in front one in the back).
Nothing special at default with sonor detecting radars.
Just tu have fun in Baltic sea..

and it would be interesting to know how much different (atomic) subs cost..

The two biggest factors would probably be maintenance and also how many would be built.