It varies a lot, but I'd say avg. $450 for an M320B.
The M520 is 8-buss architecture & the M320 is 4-buss. That's a huge difference.
The M520's EQ is 3-band sweepable, while the M320's EQ is 2-band sweep/1-fixed.
The M520's more feature-loaded than the M320,... (can't remember all the upgrades),... such as having a fancier preamp/input section,... (blah blah blah). Fancier Submix section. Fancier Aux's. 16 Tape Returns...
The M520 has a higher level of premium features and pro quality than
the M320, (sorry for the cliches).
The M520's worth a bit more than a M320/B (& usually bids higher, but "pickup-only" deals always bid lower).
Anyway, at well over 100 lbs., the M520 would be a bi*** to ship! Expensive! Maybe $125, which would be about $1 a pound! Actually, that's not a bad deal, if u think about it!

A local pickup deal would be the way to go when considering an item of this size.
When purchasing an M520, make sure it includes the rackmount power supply (PS-520) and one-of-a-kind power cable, that connects the board to the supply!!! If you don't get that supply and cable, you'll have a 114 lb. paperweight!