Well, with ReadyNAS, up until somewhat recently, you had to spend well over $1500 to get over 90MB/s including drives. Over $800 just for enclosure more recently.

Their Duo and NV products couldn't achieve those real-world rates, not even close.

The "Ultra" series in at least a 4 drive enclosure may be what can get those marks at the lowest end, but I believe only in some read circumstances, not write.

So I would say you have to spend at least $500 on an enclosure for top speeds.