Codegen 8000 or 9000 series at dealsonic or newegg.
Compucase 6A19 at directron (look also at the Prism there) and a few other places such as Coolcases.com. or Axiontech.com.
Some of the Ahanixen might work. Possibly the Athenatech 800 series or the Evercase 4252 series.
ExcaliberPC.com has their Meridian full tower on clearance but setting your budget at $50 really makes it difficult - $50. bucks and "Good" rarely coincide when it comes to cases...
. Most of the cases mentioned have 8 or more drive bays which you can rig many ways if you're handy with sheet metal. And you can get some nice removable HD brackets and rubber drive bushings from Evercase that could easly be used to modify any of the above to hold more HDs - four in three 5" bays.
. Check the For Sale section here too - you never know.
.bh.