How you recoup the cost really depends how much you drive and the type of vehicle. The more you drive the more sense it makes. It would take you years to make up the cost difference if you drive 5 km a day. But when I quoted $400 a month on gas + maintenance it was on a 20 year old Honda accord.
and yes we've had Rx330 Rx350 too, they were pigs that needed premium that's why we got the accord as a cheap beater. The difference in gas to fill a rx300 or a rx350 is minimal. Even the newer ones aren't that much better on gas unless you have a hybrid. So your fixed cost is gas which is basically how much you drive and the type of car it is. Whether it's a 99 rx300 or a 20 rx350 you still need to fill it up with the similar amounts of gas.