What implications does automobile electrification have for the supply and cost of Avgas?
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.