Not much different. On a cost per ton basis submarines are an expensive capability, especially in peace time.
There was no real gap as the German Navy set up
NV Ingenieurskantoor voor Scheepsbouw(IvS) in the Netherlands as an incubator for their submarine research and design knowledge. The technology had reached a plateau.
The classic 'u-boat' was fully developed and matured by 1914. Compare the German U43 class designed pre-WW1 and under construction in 1914 with the later Type VII refined during the 1920's and 30's:
Type U43 - 725 tons surfaced 940t submerged
Type VII - 769 tons surfaced 871t submerged
Type U43 - 65m long, 6.2m beam
Type VII - 67m long, 6.2m beam,
Type U43 - 2,400 hp 17.1 knots, 1200 hp - 9.1 knots submerged
Type VII - 2,800 hp 17.7 knots, 750 hp - 7.6 knots submerged
Type U43 - Range 9,400miles at 8knots, 55miles at 5knts submerged
Type VII - Range 8,500 miles at 10knots, 80 miles submerged at 4 knots
Type U43 - 4TT 2bow/2stern, 88mm deck gun
Type VII - 5TT 4bow/1stern, 88mm deck gun
Type U43 - Crew 36
Type VII - Crew 44
Type U43 - 164ft depth
Type VII - 750ft depth
As you can see, 20 years of development went into structural improvements following war experience to increase diving depth from 164ft to 750ft. Why? Because concealment as a defense wasn't enough, great depth was required too. The second war showed that great speed at depth was also required and thus the Elektro Boot was required. However, the high speed submarine had been developed in WW1 by the RN - the R class submarine from completely different requirements ie. to ambush submarines.
Type U43: