What are the differences between XRCDs?

I own about 30 of these babies. All in very heavy beautiful card board jewel cases. I see so many xrcds in plastic jewel cases: Japan, China, Hong Kong? I don't understand. Some of my xrcds are selling for $150 and some are selling as low as $20. Most of them, however, are all in the $100 range plus or minus $30. What gives? thanks in advance