Subs aren't necessarily such a hugely expensive option though. A single typhoon or ohio class missile boat can carry up to about 200 total warheads. That's on the order of the entire nuclear arsenal for the average nuclear power (e.g. France, China, Israel, India). Considering the massive survivability boost of a submarine, it's one of the better bets as a nuclear deterrent backbone, provided you have the technology to build such things.
It sort of depends on the latest and greatest technology to be stealthy. For example, China, by putting all of its warheads in 2 or 3 subs would be putting all of its eggs in one basket. Those subs becomes the weakest link.
US probably has the capability to track other countries' subs, but it is a little harder to do that with hundreds of always moving launch platorms. Those are much cheaper to build and if they carry MIRV warheads, they can be very dangerous. It is of course possible to see them from a satellite but because they are always moving targetting them like one target a stationary silo is very different.
