To tell a little story I was once at a farm auction that had a rural water district water tower.  It was brand new and they had just set it up on the guys property, a tornado came through and because it was unfilled at the time it got tweaked and they had to take it down, not sure how the farmer ended up with it.  It was 100 feet long 10' diameter 1/4 thick with 1/2 thick end caps, beautifully painted white and honestly looked pretty straight to me.  It ended up selling for 4k.  I really wanted to bid on it but it had to be moved in 30 days, I was racking my brain thinking about cutting it in 3 sections and welding lift brackets on it, but I knew it would take a mobile crane to load it on semi.  Now I have a semi trailer that could have hauled a section at a time but I figued it would cost to much for me to get a crane there 3 times or to hire 3 truckers to haul it.  It was probably a bad decision on my part but it also would have been a little hard to explain to the wife why I needed a water tower.

IIRC, I looked on a supplier site way back and standard load bearing ends for 10' pipe were around $4000.

Each.....

Quite frankly, I pulled the $200 out of my ass, but if I had to, I could come up with a decent number without too much trouble.  A lot of the lumber could be reused during the project (i.e. form the walls, pour, set, tear down and reuse it for the ceiling etc.).

The concept is correct though: why use a container to fulfill what is just a concrete form?  At least the lumber can be reused, unlike the container.

You can do it with $400 in cull lumber easily.
I built a 20'x20' storage building with 12' walls from the last load of cull lumber I bought for $600, and used less than a third.

I wonder if concrete might be a good solution for end caps?
A 10" pipe could have rebar welded inside, then pour a slab in each end with a form inside, and as little as dirt on the outside. I'm no engineer, but if we used a 48" cap at each end, that's a little under 12 cubic yards per end, which at $85 a yard for 3500 psi concrete comes to just under $1000 per cap. That puts a 40" at around $9000, which seems pretty reasonable. I'll bet even a 36" thick slab might be overkill.