Perhaps the clerk who priced the box set didn't inspect the Mosaic box set carefully.  Probably assumed there were 5 double CD jewel cases.  A total of 10 discs, therefore 10@$8 = $80. 
 
Ebay? $300 - $400  
Hmm... my set contains 6 jewel cases: 4 with 4 discs each, and 2 singles (in their own thinner box that also contains the booklet). 
 
At any rate, a great deal.  Not as good as mine, though.      I bought a used set (mint) from a local store in 1994... $40.  Highway robbery.    To this day I have no idea what they were thinking (6 X 7 = 42... round down?).  I bought a few other sets from them around the same time, also good deals, but nowhere near as good as that.  
Well, maybe they thought the 4 quad jewel cases were doubles, so 4x2 +1 +1 = 10?  
 
$40 -  Another theory: They inspected the first box that had 2 single jewel cases and assumed the 2nd box also had 2 single jewel cases.  Total 4 CDs @ $10 = $40. 
 
 
Nat King Cole for $40?  Maybe they thought MOSAIC was an offbeat cheap re-issue label.  (it's black and white, with no color photos or printing)