The folks really getting a price premium are those of us who get the 23L barrels, everything above that has moderate price increases for much greater capacities. I get that there is a basic cost in the fabrication and that the size is less of a factor, and maybe even easier to make as you go bigger. We each have our own needs and preferences and comfort level of value/price to get them.
You are correct, but it's still a great value even for the 23L barrels. If you leave each wine in for a month more than the one before (which lots of us do), by the time your barrel is neutral (+/- 2 years), you will have run six batches of wine through them, or a total of 180 bottles. That only costs you $1 per bottle, and you still have a great barrel that will last for many years to come and will provide micro oxygenation and concentration to boot. They're great for kits and improve them substantially, IMHO.
I have four 23L Vadai's, but as my grape wine making has increased to volumes beyond 6 gallons, I too have started to acquire larger barrels, and as you stated, the incremental cost is quite nice on the bigger boys, if you can fill them. While there is a bit more materials involved as the size increases, the production cost is probably not a linear relationship. For example, if it takes 6 manhours to produce a 6 gallon barrel, it doesn't take 50 manhours to produce a 50 gallon barrel.
And for me, there's the intangible value. Wooden barrel arranged neatly around the wine room, just look bada$$.
