I just paid $3.299 for Shell 87 Octane. I like the app for my phone called Gasbuddy. Or just go to gasbuddy.com. It shows me local prices, but I usually stick with Shell.
I talked to my dealer, and I mentioned I tried 93 octane a few times, but it didn't seem to make a difference with the MPG's. He explained it's designed for the 87 Octane, but it will adjust to what's in it. He also said, if I change what I put in it, it gets confused with what it's already learned. I didn't find out if it auto-resets to adjust for changes or not, or if it's something they need to reset in service.
Seeing as I've seen no significant difference with the few times I've tried higher octane, I've decided to stick with 87, and pretty much with Shell, or Mobil, no matter the price.