The non-RAID versions can be had for under $200 from Amazon for a single drive 2TB NAS. Dual-drive and RAID still under $300.
Here's the single drive one:
Linkstation Pro LS-VL Network Storage
You can navigate the site to look at the others versions. They all come with the "Linkstation NAS System" which is described here:
LinkStation NAS System for Home and Small Office Network Storage | BUFFALO
Here is a snippet about Media Sharing:
That still doesn't answer the question about whether it is visible on the Apple TV or iPad. I don't have mine anymore so I can't check for you (I switched over to a Mac Mini with a large attached FireWire drive. It does run all the time with iTunes though).
They do have forums that might answer your question. Since it also has a DLNA server, you could investigate apps that connect to DLNA rather than an iTunes server as well. I do know that a non-jailbroken AppleTV's don't support DLNA, but Firecore's jailbreak software does allow you to play stuff off of network shares.