I've seen anywhere from $4.39 to $2.69 and in the same state on the same day. I'm not sure why different brand stations (all large ones in the Midwest) have such wildly different prices. 

I run it anytime it's $0.50 or greater difference to regular. So lately it's been an easy choice unless you run into the ones that only have it a $0.20 spread off regular for some reason