The Sublimnos is an underwater habitat built in 1970 for $10,000. It is currently off of Point Traverse, Michigan in the U.S., 30 feet underwater. Its inventor, Dr. Joseph MacInnis, claims that it could be built for around $2000 by high school or college clubs.
This relatively cheap underwater research station makes me think of the book The Mad Scientists' Club, where a bunch of middle school boys refurbish an old navy submarine and use it in the town lake.