1. Your parent actually actively maintains a private road, and has probably done their research...
2. It costs them so much because they just have a single road, so they can't amortize the cost over an entire city or country of roadway. Public utilities tend toward monopoly for exactly this reason -- providing the utility to any small fraction of society is enormously expensive.
Make no mistake, natural monopoly is what happens when you privatize utilities. Ever met a Comcast customer who feels anything short of contempt toward Comcast? Now imagine dealing with Comcast any time you need to drive to the grocery store. Or even leave your house.
3. Parent's numbers actually make sense when compared to Florida's, once you realize Florida has an entire state over which it can amortize costs.