This summer I've been working on a project that culminated with the placement of a time capsule and the schedule was changed to Saturday for it's entombment. I didn't have much say in the matter, and since I've been with this from early on I felt rather obligated, plus it's business. You are looking at about $500 dollars worth of stainless steel boxes. That's the material cost, who knows what the fabrication actually amounted to . . .
These are a pretty standard design, and if filled with Nitrogen will keep even fairly high acid content items in good shape for 100+ years. Check back with me and I'll let you know ;)
They have a special neoprene or iso-urethane flexible seal, and then the tops are double sealed with all-weather construction sealants (Geocell in this case).