Originally Posted by Chase22
color me stupid, but why is a 3 gallon keg more valuable than a 5 gallon keg?
The soda industry used the 5 gallon kegs, and when they moved on to plastics and cardboard there were warehouses of these left over. The homebrewers moved in and bought the refurbished ones for ~$25. New 5 gallon kegs cost ~$120. Since the soda industry didn't do small kegs every small keg on the market was bought by someone for full price ~ $120, and there aren't a lot of them out there... so the ones that are being sold usually are sold for closer to manufacture cost.
As an aside, most of the warehouses of used 5 gallon soda kegs have been bought and sold, and most everyone expects the cost of the refurb'd 5 gallon kegs to rise over the coming years as interest our hobby seems to be growing at a fast rate.
TL;DR: Supply and demand