There's a whole raft of different factors going on here.
Firstly, there's the need to have a viable design. That design could be off-the-shelf, or it could be from scratch. Both have pro's and cons. If you go for an OTS design, much of the up-front NRE has been done, but it does tend to tie you in to the operating concept and supply chain the boat was designed around. It also ties you into the reactor type, fuel supply and safety protocols of that boat.
Secondly, there's the build strategy. Pressure hull rating is affected by factors like Out of Circularity (OOC), alignment of frame, bulkheads etc, as well as more mundane things like weld and plate quality. The supply chain and production QA requirements for submarines are significantly in excess of those for surface ships. It's why submarines are rarely done in distributed builds. EB and HII are doing it for the VIrginia's, but that's more an attempt to sustain two build yards than anything else. You can do non-PH steel elsewhere (we're doing that with BAES and Lairds) and you can do things like rafts and decks to be inserted into the PH units in the build yard, but it will be very tricky to try and do multiple PH units in different yards.
Then there's the supply chain. There are literally tens of thousands of equipment items on a boat, from simple things like fuse panels, display screens, junction boxes etc, through valves, pumps, chiller plants, flexible couplings, electric actuators, hydraulic actuators, to main gearboxes, turbines and so forth. Every single one has to undergo a significant level of design, qualification and testing - let alone production QA - before it gets on the boat. Many of which are non-trivial and can incur quite a bit of cost, which is usually why you see "inflated prices". It's rarely a question of just picking one out of a local supply catalogue, because if it doesn't match or exceed the requirements of the original bit of kit, it compromises the safety argument for the system. There won't be many equipment suppliers who qualify their products for submarines "just in case someone wants to put one in". This is often the hidden part of why operating submarines (and particularly nuclear ones) costs a lot of money.
Then there's the manpower - to design, specify, QA, build, accept and operate the boats. Again, the particular nuances of submarines - especially those with a kettle - are not the same as those for ships. One can reduce the direct need for specialists by using an OTS design, but you need an indigenous expert design and operating authority once you've got the boats in service, so it's still a stretch.
All of these things can - and I hope will - be done. But the timescale and budget will need very careful balancing and consideration.