Can you name one of these "can't melt down" designs that is more than just an unproven napkin sketch?
All the ones I've seen that are at least trying to build a prototype rely on having a cooling pool, meaning they absolutely can melt down if that pool is drained.
That means you need to protect the entire pool as well as you would protect a traditional reactor, and have an emergency supply of water on hand. So containment buildings, leak proof pools that can survive earthquakes and other natural disasters, all that stuff. Doubtful there will be any savings over traditional designs.
There are some types which are more likely to be actually meltdown proof, although even then you need to protect the reactor from physical damage. But they all have severe problems with the reactor vessel being damaged over time by the fission reaction inside, and every attempt to build one has been an expensive failure.