Well if you are playing GWX I can say for sure that there are no upgrades you really need to worry about. The XXI comes with state-of the art everything in its hefty price tag of around 20,000 renown.
As far as torpedoes are concerned you will pay 1000 per zaunkonig 2 and 800 for the zaunkonig 1 I believe. I couldn't tell you what the FAT and LuT series torps will cost as I never used them but I would guess 300-500.
On the bright side your boat's initial loadout includes Zaunkonigs/Falkes and /or Lut and FAT torpedoes which should alleviate the cost a bit.
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