I don't typically look for cheapest gas prices since I have noticed that not all gas is the same in my daily drivers. We have Kwik Trip and Kwik Star gas stations around here and I used to think they were the cream of the crop because they had "Top Tier brand gas" and a warranty that says if their gas causes any mechanical issues, they will cover the cost of the repairs, but I stopped using them because I noticed about a 10% drop in gas mileage from them versus a few other gas stations in my town that were 5 or 10 cents more per gallon.
A few years ago, when we went to Iowa, we filled up there. They didn't have ethanol in their 87 gas, and we noticed a bump from the usual 28mpg that we got in our 2012 Fusion 4cyl, up to 32. If E0 didn't cost so much more due to regulations in my state, I would prefer to use that, but most places only offer E0 in premium 91 octane or higher, so its about 75 cents higher than E10 87 gas. Some gas stations use higher or lower percentages of ethanol in their E10 gas to cut costs and I can usually tell a slight difference when they do.
If its a place I use regularly, I will test the percentage in a glass graduated cylinder. If you add 100ml of gas, add a little bit of water, the ethanol will separate overnight and you can measure how much actual gas is in it.
This video only uses 10ml, but I do 100ml as mentioned because its easier to see and more accurate measurements. You can technically use the gas after the test, but its a lower octane due to the separated ethanol, so I only use it in my lawn mower.
https://www.youtube.com/watch?v=d8pV-x9uLiE