The price of anything is dependent entirely upon supply and demand.
There is relatively small supply of smaller coaches. I personally drove 1500
miles (each way) just to LOOK at our 25' Sovereign
. From about 1970 (or probably a few years before) until the advent of the new International and Safari lines in the late 90's there were simply a much larger number of the 30' + units made than the smaller lengths.
Now, of course, there are a lot of people out there discovering Airstreams, finding a disappointingly large discrepancy between the price of a new one and the available funds, and searching out used models. Naturally, they want something they can tow easily, so the search for 25' and smaller Airstreams at affordable prices has become harder and harder.
Limited supply, lots of demand, higher prices result.
Now, why prices are so much higher in California particularly? That appears to result from particularly high demand, but it is not clear to me why this is so.