Lol, well I made a mistake in researching. Their gasoline models are affordable...but they get the same mpg as my foz. I was looking at the epa of the diesels which are prohibitively more money ~10k$ for the same years versus 4k$. So nm to Volks, at least for now.
As for other options here are my other considerations by researching mpg alone:
Nissan Sentra (200sx)
Note: I realize that there are some sketchy ones in there, but I am trying to be objective. I'm trying to stack them up by mpg, initial cost, maintenance cost, and longevity; all in the aim to get the most economical car, not just best mpg.
Also founda better mpg comparer: Search by Manufacturer
First of all, the EPA mileage comparisons are not valid. Cars often get much worse, or in some cases, better, mileage than the EPA ratings. For example, my old '98 Acura TL was rated at 20/25 and routinely got 23-25 in mixed driving and 27-31 on the freeway, whereas my wife's Sable was rated at 19/28 and got 19-20 mixed, maybe 26 on the freeway. Ratings are a general guide only.
Second, if you are looking at $4000 cars, the possibility of repair costs is very high, and repair costs can dwarf gas mileage differences. This is like buying a hybrid, getting 10 mpg better than a non-hybrid for 100k miles, and then having to replace the batteries for $5-8000 and canceling out every cent of fuel savings.
Likewise, if you have a $1000 surprise repair on any car in your list, it will kill your fuel savings dollars for 3-5 years over something that uses slightly more gas. Look at total cost more than gas mileage, especially when looking at old stuff. A $2-3k engine or transmission replacement completely takes you out of your price range.
In your price range, you are right to come up with a list of acceptable cars and look for one at the right price and in the right condition. The Prizm/Corolla would be fine, any Honda Civic would be fine, how about Mazda Proteges, etc. I'd be biased against Mitsubishis and Suzukis because of parts availability, biased against Saturns because of reliability, etc.
Look at some Consumer Reports used car reliability ratings and factor those in too.
Lots of luck,