A lot of this seems to be consulting work and not actually software cost. How are they supposed to estimate the necessary number of hours if you haven't told them what you need.
If you are serious about it, identify a few companies (no fewer than 5) and send them a Request for proposal (RfP) with what you are looking for and let them bid.
Dude, if someone here is "well akshually" that is surely you. When people talk about people on the internet that are annoying to meet - that's you.
But apart from that. You are simply mistaken in a lot of things or are projecting so hard you may as well have an HDMI input.
Nobody said 50s era or even modern diesel subs or their other non nuclear equivalents are "better" than a nuclear sub in all ways but in some situations, e.g. coastal defense and operation sin shallow water, they may be better suited to the mission than a nuclear sub that is 4 times as large.
In addition there are economical considerations. If I can buy 4 diesels for the price of a nuke sub it may be better for me to have 4 diesels who can lie in wait at 4 places at once.
The question is mission fit of the asset. A ship will sink all the same whether it was sunk by a 2 billion USD Nuke sub or by a diesel on the way to the wrecker that had a really really lucky day.
They are loud when they recharge, they are slow compared to nuclear subs and they carry much less armaments.
On the other hand, when they are on battery power modern diesels can be much quieter than nuclear subs, they are much cheaper and smaller so ideal for operations in coastal waters. Which is why many (also western countries) rely on them for coastal defense.
Economics wise: You can trade 3 diesel subs against a nuclear subs or a large warship and still come out ahead cost/effort wise.