Based on a quick compare, it looks like VXA2 is probably your price performer. Assuming you want to back up one TB, and assuming you actually get full compression on the tapes, VXA2 is your best bet. In US prices, it comes in a little under AIT2 and DLT4 and significantly under LTO1 and AIT3. (And way below LTO2 and SDLT320.)

In each of those jumps, you get a jump in tape capacity. Basically 80GB vs. 200GB vs. 400GB. This means that you have to swap tapes less often. Obviously.

I did stumble upon a 10-tape VXA2 library for about $1890, which seems like a good deal. That's only three times the cost of a single drive, is external to boot, and raises your drive capacity to 800GB, which means a lot more unattended backup.