To really get an answer to your question you'll need to give the community a little more information. Not only the physical specs that you require but also what features are required.
Interconnect type (Ethernet, FC, etc and what speed and how many ports)
Form Factor (does it need to be rack mount or stand alone)
What sort of redundancy do you need? Power supplies, controllers, Multi-Path IO
Do you need snapshots?
Do you need replication?
How are you planning on backing it up?
With that said even the entry level Enterprise single controller iSCSI SAN solutions you're going to be looking at $10k+ for 2-5TB of storage. That's just the where the market is. If you're running 4 servers off this one SAN just remember that is a single point of failure that could take down all the services that those servers are running. So you need to plan accordingly.
My favorite storage platform for do it yourself these days is OpenSolaris. With ZFS you get a lot of features only found on the commercial SAN solutions. There's also Nexenta which is commercially supported storage platform based off OpenSolaris and ZFS. It may be a good compromise.