They are pricy because they are all metal and are rather well built. Also, if you manage to solve one you can then enter “The Isis Adventure” where you could possibly win some money (it’s far from easy to do so, you have to solve a very difficult riddle and may have to do some traveling).
The Isis puzzles are hidden mechanism puzzles which means to solve them you have to figure out for yourself what is going on inside them. There are some spoiling youtube videos out there which will show you how to solve them. If you are serious about wanting one then don’t watch those videos as it will ruin the fun of solving one for you and you would have wasted quite a bit of money on them.
Also, have caution if buying a used one. These puzzles have serial numbers and you register them. You also register a different number that you find inside when you solve it. This is what allows you access to the riddles/games/prizes. If you do buy a used one make sure that if it has been registered that the seller is willing to transfer that registration. And if it has been solved make sure that it has been put back together properly (it is possible to do this incorrectly leaving the puzzle unsolvable).
The Isis series is among my favorite puzzles