I'm working on them at the mo - I owe TTony a box of assorted stuff (it's in production, honest!).
They're surprisingly tricky to make - you've not got much area to secure them to a spoilboard so it's not uncommon for the milling bits to rip them free - one dead ring. They also take a fair bit of time - the blank wood has to be cut and planed square before it can go on the machine and it has to be thicker than you want the ring, which means that once it's "done" you still need to get it down to the right thickness. At that point, though, it's pretty thin and fragile. It gets even worse if they're going on a carve top guitar.
$150 does sound a lot until you work out just how much time is involved and what the wastage rate is.