Hi there, I bought a cx300 when I sent my ER4P->S for repairs. While the microphonics were greatly reduced, I found the sound a huge step down. In part it was also because they don't seal as well. I haven't heard other canalphones in the same price range but think that IMHO, cx300 wasn't a huge step up from cheaper phones such as mx500 but that's me. YMMV tho : )
Oh yes, I have seen it available for about $110 (can't remember if it's ebay.com.sg or yahoo auctions tho)