Root Cellar - the author said it cost about $1,000 for this 8x8 foot cellar: concrete blocks were $1 each at the time, and he used about 320. About $500 for concrete and mortar, and the remainder for rebar, wood, etc. He contacted a local gravedigger to use a backhoe to dig out the space for the cellar.
A really simple way to to use solar air heating collectors to heat the soil below a high tunnel greenhouse. While a fair amount of excavation is involved, the system is really simple and cheap if you have access to a backhoe. (which I totally do)