They are ridiculous in price. The Lamb leather LCD pads are the same price. I remember putting Annie pads on the K501 and also switched the foam over and it was wonderful sounding, I don't think it's worth getting Annie pads if you have the 650. If you buy some extra foam, cut a hole the size of a 5p on both sides so the driver shows. Take the back grill off and get something small to poor out the back foam gently you can make the 650 a bit more airy, defined with a few db less mid bass and it sounds really good, I still prefer the warm goodness of stock though.
I'm in a similar boat but with my PC. When I added a GTX 960 sound card to the build came to like £870 and that's without a good graphics card, although I do have my Mixamp + Mojo so that can wait I guess. Things are getting expensive so no room for more headphones although there's not much worth looking since I've blacklisted Audeze haha.