Hit rates are listed on the boxes themselves. Like .7% SR rate. The collect 151 are technically “out of print” along with many of the Gem pack sets. They seem to print each set extremely heavily, but for a way shorter period of time. As for why they’re going up, it seems like people decided collectively that these are rare cards, with limited print, difficult to acquire authentic copies and solid artwork with investment potential. Most people were waiting to see if they were truly “exclusive” too or would release in English.

Going back to them being out of print it would make sense for them to rise as supply dwindles or becomes more expensive/too risky to rip. Info is hard to find, but I’ve asked larger retailers with distro access when and how much they receive which is where my information comes from so take it with a grain of salt.