The only way you're getting 3-500MB/s on a 4-disk ZFS RAID5/raidz is with very, very fast SSDs and a very, very fast CPU. Not exactly "very cheap". (The compute and I/O overhead for raidz is significant.)
At best I can get 60m/s out of it. Each drive can do about 100m/s sequential but that is rare.
Putting an ssd on for caching read/writes though really changes the calculus of this.