What implications does automobile electrification have for the supply and cost of Avgas? The electrification of automobiles has been picking up pace recently, with various countries and manufacturers setting targets for an electric- or electric/hybrid- only future.

If these plans come to fruition and are widely adopted, the demand for gasoline, and therefore its supply, would decrease considerably. While avgas is something of a specialty product, I would guess that it shares the bulk of its components, and their manufacture, with automobile gasoline.

Have there been any studies of the consequences of this change on the inflation-adjusted cost and its supply of avgas (though I imagine it will be available at some price), and also on motor gas for those aircraft that can use it?
I have not seen any studies on this, nor would I expect to as Avgas is such a niche product and the price of it is not heavily dependent on the price of the underlying fuel itself. If you look at this answer the reason avgas is so expensive is that it has special requirements for making it, storing and transporting it and so little is produced that there's no economy of scale. 

In any case your question is based on the premise that gasoline production is going to decrease. Even with the rise of electric cars most oil companies aren't forecasting a drop in gasoline demand until the 2030s, maybe even the 2040s as the increase in demand for new cars will oustrip the number of electic cars for some time. As a decline in consumption does not guarantee a price rise, any forecasts of this kind would be premature, and not likely to be reliable.