Cheapest AVGAS we have within 100 miles of here is $5.03 a gallon, with the average being around $5.50, while regular 89 octane car gas here is averaging around $2.90.
Running high octane fuel in a low compression engine, such as a chainsaw provides no additional value or power for the engine. High octane just helps prevent early detonation in high compression engines.
Stihl recommends 89 octane gas.
Add to that your own fuel costs for driving to an airport that will sell you AVGAS Vs. driving to the closest gas station and, unless you are like me where the airport is only a 4 mile round trip, the next closest airport is a 40 mile round trip that will cost 3 gallons of car gas in my Suburban, so, an additional $8.70 for fuel, while the 7/11 is less than a mile.
A at a 50:1 mix, that's not enough oil for many 2 stroke machines, as many require 40:1 ratios. That mixture may cause premature wear on an engine that requires a lower fuel to oil mixture ratio.
To summarize:
10 gallons of AVGAS will cost $55.00 plus $8.70 to transport it = $63.70. (plus wear and tear on my vehicle)
10 gallons of car gas will cost $29.00 plus 34 cents to transport it = $29.34 (plus wear and tear on my vehicle)
A bottle of Stabil costs 5-6 dollars on sale and will treat 20 gallons. That's the way to go.