I would imagine the cheaper drives on eBay will be the ones with the Fibre Channel interface, because most people will run a mile on seeing that. So I see a 48 slot library with two LTO4 drives going for 2000USD.
The trick to realize is that even dual 4Gbs PCIe FC card can be had for buttons, and the drive an library will work perfectly well in an arbitrated loop, no requirement for a switch and OM2 LC-LC cable is also cheap.
New LTO4 tapes are ~30USD labelled, I reckon on 1:1.25 compression ratio so that is around 50TB of backup capacity, not bad for under 4000USD investment.
The next trick is to realize that in a few years you can swap out the LTO4 drives for LTO6 and keep using the library...