A sub is built to withstand a 1,000+ atmospheric pressures and to support a crew of over 100 for months on end without ever surfacing. A space station module only has to withstand 1 atmospheric pressure and the station support a crew of a dozen or so. There is no way a station module should cost more than $50 million a piece with the total cost for components at more than $1 billion or so.

The most expensive part of a station should be (by far) the launch of the components. You say it would cost around $3 billion, I'm willing to put it at $6 billion just to be cautious. That's still comes to only $7 billion, or just under the cost of 1 1/2 Nimitz class carriers. Such a station would be far less expensive than the $60+ billion ISS and a hell of a lot more useful.