Another underrated small car was the geo metro - more than 50 mpg in the cheapest configuration, and very reliable (the early ones were made in Japan, and could easily make it to 200k miles if the driver wasn't very heavy).
That said, in the USA you would almost certainly be better off buying a ~10-15 y/o honda or toyota - slightly worse mileage, but probably much more reliable.
I drove a Suzuki Swift (same car as the Metro) for a decade. Cost me $10k (Canadian back when they were cheap), never spent a cent to maintain it. Gas, I never noticed actually spending money on it ;-) It wasn't the cheapest configuration, though, it had 4 cylinders (as opposed to 3) and 14" inch tires instead of 13".
That said, in the USA you would almost certainly be better off buying a ~10-15 y/o honda or toyota - slightly worse mileage, but probably much more reliable.