I interned with a group at NASA Langley which is working to use mostly off the shelf components to build a portable cave. They already have a more than working prototype, or did when I was there 2 years ago.
Unfortunatly I do not remember prices, but I know for sure this was maxing at around $30,000. I don't have time to go hunting now, but here is a good jumping point: http://develop.larc.nasa.gov/projects/
Just google for nasa develop and cave