Promise makes a lot of multi-drive 2 and 3U enclosures... SATA, SAS with iSCSI, SAN, etc..

http://www.provantage.com/promise-technology-vj1830~4PROM1CF.htm

On the cheapy side, they actually have a 4-drive direct attached raid enclosure for 4 drives at just over US$300. Though one review I've read claims it's very slow (running some real-world simulation benchmarks which I don't really trust)

I'm still leaning toward the DroboPro myself because I really can't afford something I'd like more. Like a 12 or 16 device (high quality) enclosure connected via fibre running SAN to my server... That's probably the only way to get over 100MB/s read/write speeds.