Most pro level units for raid or NAS on 4 bay units start at around $800 without drives.... Something with speed and better management will not be cheap. Don't be surprised to see them in the $1500-2k range for more bays with drives.
Drobo, is like the ones from Dell, or Netgear....rather limited, and slow. Like mentioned above, on RAID, you dont want to use more than (75% is my limit) 3/4 of the drive space.
Ones from ReadyNAS, Synology, and the pro level stuff have built in OS's that manage everything, and are very fast as well as easy to use. Also, depending on environment temps, I would keep these in proper temp location, as they can get warm with lots of read/write. Sometimes it is much worth paying a little extra for server level drives vs basic retail home use, or "green" drives. WD has some 1TB drives that are better rated for 24/7 on use, although the black caviar drives are decent so far. If you can build a NAS(or have someone do it for you) with FreeNAS using an old PC setup, you will have the highend performance for very little.