Here's some info for green houses to give an idea of the kit costs. The cost is from $20-30 per sq. ft for the traditional onces, rated at 80 mph (not 4 seasons):
http://www.canadagreenhouses.com/
Geodesic kits for 4 seasons:
$20-$25 per sq. ft US
http://www.geodesic-greenhouse-kits.com/sizes.php
The 4 seasons don't seem to be much more expensive. They have an insulation value from 1.8 to 3 and heat storage from the sun during the day.
To minimise cost and also the issue of heat retention, this can be integrated into a living structure for a circular type building with a "greenhouse" cover over an interior court area. Another smaller structure alternative is the "earthship" structure where the front is used as a greenhouse.