They offer them individually, so those who use certain sizes mostly (e.g. 6mm or 8mm?) could just buy one or two for use on the rare occasions that they're needed. (Note: One can actually use just a regular domino of the desired size to do the job of those markers in finding the mortise center point, but it won't be as quick. I did it once.)
I just noticed the Domino sled (on that same link)
It looks pretty cool, but it looks like they missed out on an opportunity? They could have used a pin to locate it, similar to the LR32 plate. It may not appeal to everyone, but at least an option for those who do have the correct rail.
I have a 500 and it cost me $130 AUD shipped for one set. I don't consider that unreasonable. I guess if you've got both Dominos then that would make it expensive but I definitely could have used these on a few jobs in the past where I couldn't reference off flaps, pins or fences