I bought them on Amazon for $20. I found they were absolutely worth the price not so much for the isolation but for the shape of the pads. My desk is pretty short, and they come in doorstop-shaped packs of 4 so I used them to tilt and elevate my monitors at the same time. It's a really great feature, and worth it IMO. FWIW I also tested them against some other materials before the pads shipped and there was a noticeable difference in the bass isolation especially.
I have those, they do a good enough job in separating the monitors from the desk. Not 100% mind you, you may still need stands, perhaps filled with sand and with small points of contact on the bottom, if you're cranking the monitors up..
I find them pretty useful both for directioning monitors/tweeters and to tighten up the bass response. Definitely recommended, although I might switch to Primacoustics Recoil Stabilizers someday.