I wish the 3 gallon kegs cost less. But they cost just as much, and more in some cases, than the 5 gallon kegs. It's crazy. I usually put in the comments of the order to make sure the posts can be removed before shipping.
The difference in the amount of steel is minimal. I'm sure most of it is labor. If there was a price difference, it would be very small due to materials and freight.
New 3 gal kegs cost less than 5 gallon kegs. Small difference in price. There is the same amount of parts, just a little less steel, the number of welds is the same. The labor would be almost identical.
In the used market the high cost for the 3 is due to the low number that were made. I have seen some large used cornies, 15 gallons, and the price on those was very high due to the rarity.