Right, even with "old" technology as you put it, the costs of construction from digging a hole to getting online are still a big question. Nobody really knows exactly how much it will cost.
Skip the intro, lecture starts at ~5:00. Goes over pretty much everything, from the research history, to the actual technology, to how it compares with existing reactors in safety/efficiency/output/environmental impact. I won't bother to explain any of it because you really should just watch the video (and he explains it better than I could anyway), but the main difference to take away from this compared to other "huge technology leap" solutions is that this isn't actually new. It's already been done decades ago and already been shown to work; it was stopped mostly for political reasons. It just needs to be modernized and continued to be worked on so it can actually be put to commercial use.
Both videos were far more convincing than the first one that was posted. I'm even going to say they were fun to watch, since the presentation is very funny, and still instructive. The only thing I didn't like is how much time he spent bashing the current reactor, i would rather have a video where he focus on explaining thorium reactor instead.
Most of those are using solid fuel. All of the benefits from this idea is using it in liquid form dissolved in molten salts (hence the name molten salt reactor, or for this one in particular Liquid Flouride Thorium Reactor - the LF is as important as the T) which, for various reasons pointed out in the video, solves a lot of issues that exist in current reactors.
The only molten salt model on that list was from oak ridge, which was just a test reactor as a proof of concept when they conceived of the idea. Since it was a simple proof of concept, it also didn't include the entire design, just the most critical parts to show that the idea was sound. The design worked fine, they just didn't get the money to continue for various (mostly political) reasons, and it hasn't been pursued since. That's the point of trying to draw attention back to it again and revisit the idea