Seconding this. I work with a lot of geophysical data, and there's a reason why our library is stored on LTO.
Once you have the infrastructure and supply chain for it, there's simply no cheaper way of long term storage per TB. The drives can be pricey, depending on which you use, but the standard IBM tapes are pretty cheap.
I don't know, we usually buy in bulk. I tried finding the invoice we got after a pallet of 15TB tapes, but I can't seem to find it.
There are also different tape types depending on which capabilities you need, which of course affects the price as well. We use a few variations on the IBM 3592 tape, but most of them are WORM, and in a tape format that "anyone" can read.