Interesting question. In the early 2000s they started doing 'mini temples' which were basically just chapels. As far i know they fizzeled out, but I wonder if this move was in part due to the high costs of sustaining/maintaining full temples.
There was a prophecy floating around, that the church was never going to lose another temple. I think it was given sometime after Salt Lake was dedicated. Well, that lasted up until that one temple in the Pacific burned down to the ground. Now they don't talk about that prophecy anymore, since it is now a failed one.
I wouldn't be surprised, if they eventually lose one or two of the African temples. I understand the one in Nigeria has all but been closed up, due to both violence and a lack of interest in the area. However, I would not be surprised to learn if squatters have taken over it. I do know enough about buildings here in America, to know that if a building constantly sits empty and abandoned long enough, a homeless person is going to start camping out inside, and it would not surprise me if this is a secret problem of Mormon temples. Building them in nice suburban neighborhoods probably helps against this, but even nice neighborhoods increasingly have transients.
I don't think they ever would. Image and their obsession with displaying the "growth" of the church makes me think that they'd rather let a temple sit unused than swallow their pride and sell it. They'd rather eat the opportunity of cost of keeping their precious temple trophy on display (at the cost of the members of course) than sell it off. If they DID decide to sell it there would definitely be some kind of faith promoting spin given. They would say something like the area has "matured" in the faith and the need the funds to build temples in areas where it is growing "so fast" that they need funds diverted to these new areas so they can build even MORE temples!
Unless I'm mistaken, they attempted to sell the original Nauvoo Temple to the Catholic church. Brigham wrote a letter to them on Oct 31, 1845. See History of the Church, 7:508-509.
I reckon with some of the temples if they can't afford them anymore they'd sell some of the land around it. For example the London Temple is fairly small, but there's LOADS of land around it which I assume the church owns. There's enough room to probably build a small theme park there.
I reckon if they built houses on a temple's ground some TBMs would pay a large amount to live there.
I served a mission in Chile 2006-2008 and there were several chapels that had been locked up and basically abandoned by the church. Weeds growing all around, windows smashed.