The current list is USD 229 at Adorama, but apparently they go in and out of stock fairly regularly. They list for AUD 399 here......
Rover, my M6 is a TTL .58. Really really sweet with a 35mm (which is my prime lens) and also great with a 28. But the 90 frameline is pretty small, which is why i was considering the magnifier. I find I use the 35mm for about 80% of my shots, and the 90mm for another 15%. Remaining 5% is mostly 21mm.
The .58 finder is usable with the 90, but kinda only just.
On my camera, with the finder magnifier on, the .28 lines are next to useless, you have to crane around to see the 35 lines, the 50 lines are just the right size (about like the 35 frames without the mag), the 90 lines are quite usable. Which is pretty much exactly the same as a normal .72 finder.
But as Stuart said, the Rangefinder patch is much bigger relatively.
It kinda gives you effectively 2 finders in one.
But...there's a big difference between trying it on the camera for 10 minutes and actually using it. And it's a fair chunk of money for something you end up not using.
tim