The high octane fuel is not so cut and dried. At some fuel stations, the only fuel without ethanol is the premium fuel. Depending on your car, you may not want ethanol in it. And some vehicles (like mine) get horrendous fuel economy with ethanol blend fuels, that makes premium the better choice (if it is the only one without ethanol).
The biggest cost of a new car is depreciation, which trumps fuel, insurance, gadgets, etc. So the best way to not waste money on a car is avoiding a new one.