Browning shifted production from Belgium to Japan / Miroku in 1976. The Japanese guns are set up for choke tubes, and the barrels are thicker-walled. As a result, they tend to command a bit more money from most folk (altho there are folk that prefer the slightly lighter Belgian guns).
Price depends on where you live, I guess. Places with upland birding will favor the 20ga (even a magnum) while places with lots of waterfowling will gravitate towards 12ga guns (if they even still use the Auto-5). My last two (2) 20ga Auto-5s (pre-panic) were bought in the NoTexas area and sold in the $600 -$700 range.
The 20ga magnum was always a bit of an odd duck, if you pardon the pun - it's heavier than the regular 20ga and most upland hunting doesn't require magnum shells, while it still doesn't have the payload capability desired by the waterfowling types (especially once non-tox shot became the norm).