Mike,
I'll paraphrase my cousin the petroleum engineer, so this will sound like a fairly simplistic answer but it is the accurate answer.
MoGas (motor vehicle gasoline) is a mass-produced and mass-consumed product, subject to the supply-demand fluxuations of a market and consumer economy.
Avgas is produced in extremely low quantities compared to MoGas, and only at a very small handful of refineries. It is considered a boutique fuel in the petrochemical industry. As such, it's set output and consumption is more consistent, and thus, so too its cost and price. She said the prices ought to drop a little, but not in pace with the more active MoGas consumer market.