So just paper napkin math here, let's say you drive 20k miles a year and the cost differential between mid and high octane (US 89 and 91/93) is $0.15/gallon. If you average 23 mpg, you'll spend a shocking $130 a year extra putting 91/93 in. That's less than $11 a month, or thirty six cents a day. If this is too much, you will be better off on the bus.
Of course if you get better mileage and drive fewer miles per year, it's an even smaller amount. Just running numbers on my wife's habits, of 10k miles a year, getting around 17 mpg average and keeping the car three years, we'll spend about $265 total putting higher grade octane in her car before we sell it.