My Mazda has a 15.5 gallon tank, I generally refuel at the quarter-tank range (the gauge mark is way off though) and it's around $33 for that. I always try to fill up so I can gauge my mileage (calculating the amount of gas I just put in - and thus used - against the trip odometer which I reset at every fillup), which has been floating between 23 and 25mpg lately (consistent for my 4-banger: 21 street, 28 hwy).
Premium gasoline (91 octane) isn't generally needed for a modern engine unless it's a higher-compression engine, mostly found only on German cars, where they get more explosion out of the gas in a tighter area via more precision and need that added extra bit to avoid knock and misfire. If you're putting it in your car and it doesn't say "premium fuel required" on the dash and/or fuel door, you're wasting your money (some cars say "premium recommended", like Jeep/Chryslers, there you can use regular 87 octane to save money and you'll only have a minuscule performance reduction). Now, were we talking about eking every last nanosecond out of your zero-to-sixty times, then it'd matter, but for the average SUV, your 0-60 times are embarrassing anyway and you won't notice the performance drop.