Out of those you've listed, I've heard or owned the MX500, PX100, numerous Sony MDRs over the course of almost two decades, the KSC-75, and the Portapro.
- Most Sony MDRs really suck with a few exceptions. In terms of the few exceptions, they're unfortunately out of your price range by quite a lot, and most people would still prefer other similar priced headphones in those high level priceranges.
- MX500 is decent. But I dislike earbuds, and the KSC-75, PX100, and Portapro are better. They're noticeably better than any earbuds that come with portable CD players or MP3 players
- KSC-75 is pretty good. Inexpensive, quite portable, sounds rich and enjoyable for most people. (The now discontinued KSC-35 is better from what I've heard from many people, however.)
- PX100 and Portapro are good, solid headphones. They're at the upper limit of your budget. The PX100 is more laid-back, while the Portapro is more engaging. I personally think the sound of the Portapro is more enjoyable, but the PX100 seems to look better in my opinion.
Good luck!