All except SCEJ discontinued production of the model 50000 and older. SCEJ only keeps it in production because Japan has more HDD support and SCEJ is actually interested in supporting the HDD, unlike SCEA and SCEE.
The used systems are selling for more than the new slim systems because people know they aren't made anymore and if you want HDD support, you have to buy the old model, so people have little choice but to buy used, and at whatever price sellers set. Now that the supply is limited, the prices are increasing.
My PlayStation2 HDD guide.
NTSC J SCPH 30000 with HDD and PSBBN 0.32
NTSC U/C SCPH 30001R Astral Blue LE PS2
Final Fantasy XI status:
Server - Ragnarok, Race - Mithra, Jobs - BLM 72 WHM 51 SCH 39 SMN 25, Nationality - Windurst, Name - Bungiefan