Yeah it's possible he's overstating the case, although he seems to have gone in to quite a bit of detail estimating the costs. But you'll see from some of the comments above those might be distorted.

My own very crude back of the envelope, for replacing all worldwide coal generation with AP-1000s (at $7bn a pop) puts the cost somewhere in the $6T range. This is assuming I got the unit conversions correct...

As for MSRs, there's some very interesting history there. There is, of course, the fact that MSRs weren't as good at producing the necessary byproducts for making nuclear bombs (remember this was during during the cold war). But the other accident of history was simply that Oak Ridge National Labs (who ran a prototype MSR for a few years) just happened to be in the wrong part of the country: the story goes that Nixon apparently wanted to shore up political support in (California?...) and so pushed money more in the direction of the LMFBR.

Nixon also fired the director of ORNL too, apparently because his support for MSRs and advocacy for increased nuclear safety didn't gel too well with that administration. At that point MSRs were kind of forgotten about...