I've worked for several production companies that have similar or larger archives (one was well into the Petabyte range). LTO is the way to go. It is the cheapest option for very large archives, and if the tapes are properly stored, they last a lot longer than hard drives sitting on a shelf.

The real way to do it is a tiered archive, where everything goes to LTO, you have more recent media (1-2 years old, depending on project length) on hard drives, and current media (still in use + past year or so) on a NAS. LTO is still your primary archive; everything else is for easy access to media you're more likely to need now or in the near future.