If DVDs, even dual-layer, aren't high enough capacity and 100+GB tape drives are too expensive, perhaps just buying more hard drives is your best solution. Add the drive, clone data to it, and de-install it. When done, off-site if possible.
A less expensive tape option would be AIT2 or 3. AIT2 is 50GB native. Drives can be had for under $1000 (even cheaper from ebay) and tapes are around $40 each.
Personally, I've decided that my media, which is a measily 500GB or so, is not valuable enough to back up. I'm burning it to DVD to organize it and free up space. Nothing is really being duplicated. If I lose it I'd be disappointed, but it wouldn't kill me by any stretch. My valuable files -- Word & Excel stuff mostly -- I'll back up on occasion.