Making cars is never cheap. Producing a more advanced vehicle, like our hypothetical 80 mile-per-gallon gas-sipper, even less so.
It's widely known that Honda lost money on every first-generation Insight it made. That's not really surprising. Not many were built, it used brand-new technology at the time, was made on a specialist production line and was constructed from expensive aluminum rather than pile-it-high steel.
Modern car companies are less willing to experiment at a loss, so any car combining the latest fuel-saving technologies would need to make money. Building a modern remake and incorporating the latest technologies simply drives up the cost.
To date, the best rating for a gasoline-powered car is Honda's CR-Z coupe, which gets 37 mpg. It costs less than $20,000, but Honda has struggled to maintain 5,000 sales per year.
One factor in the CR-Z's high price for shipping it from Japan. If it were manufactured stateside, could bring costs down, but there's little incentive to build a truly niche vehicle like that on U.S. shores.