You need to ask yourself why you are doing this.
If it really is all about the money, then those have have mentioned a good, small, used car have got it right. My favorite for this sort of duty is the Toyota Corrola. The ones that have been made for the last 5-yrs+ have been EPA rated at 40 mpg. Toyota quality is legendary. Heck, their gas cars often pile up reliable miles like a diesel would.
When I needed some basic transportation, I bought a used (1988?) Toyota Corrolla at its 50,000 miles point. I think I put another 30,000 miles on it and then sold it to a friend. The daughter of that friend is still driving that car today (last I heard). It still looks more or less like it did when I owned it which was more or less like it was when it was new. Great, basic, low cost transportation. It was compfortable, roomy (for a small car), drove nice, handled nice, rode nice (again, for a small car) and wasn't offensive to look at.
So... if it is all about the money, there ya go.
Big iron raised an interesting point; fuel availability (or lack thereof).
The big gas lines of the '70s (I was 23 at the time of the Arab Oil Embargo), were caused not by price, but by availability. Later, it was simply that the Iranian revolution (when I was 29) kept their crude off the market because of political chaos (it wasn't withheld ...the facilities were simply off-line).
During both of these times, you simply couldn't count on being able to get gas when you needed it. Range became extremely important in car ads. Cars started being built with really big gas tanks. Why? Because you could put together a full tank of gas bit-by-bit over a week or so just before you left for a big trip to Grandma's or whatever and if you had a big tank, you could better count on being able to "get there and back" if you had really good range.
Hmmm... availability and range. What to do?
I'll make a plug for a used diesel for you (VW Golf, Jetta or "New Beetle").
My 2003 VW Golf TDI (diesel) has a 14.5 gallon fuel tank. It is EPA rated at 49 mpg. I actually get ...on the highway, at 70 mph, air conditioning on, in the summer, car fully loaded ...49 mpg. It is one of those rare vehicles that can actually achieve its EPA highway rating in the real world under real world conditions.
My highway range is 14.5 gal x 49 mpg = 710.5 miles.
There is an internet forum for owners of TDIs; Fred's TDI Club. Here is a thread about the "600 mile club" (getting over 600 miles on a single fillup). It is quite common to hear of TDI owners getting 50+ mpg on the highway.
http://forums.tdiclub.com/showthread.php?t=126336&highlight=800+mile
Honestly, some of the guys on Fred's are "mileage nuts".
Fuel availability raises another issue.
With a diesel, if you wanted to, you can even
make your own fuel. Hook up with a local restaurant or organization that does a weekly fish fry, ask for their used fryer oil (most have to pay to have it disposed of) and then convert it to biodiesel in your garage. It costs about $1/gal for the ingrediants to make your own fuel. I know two guys that used to make their own in 5-gallon buckets. Most people don't want to fool with this (I don't ...I buy my biodiesel factory made). But if fuel were suddenly hard to get... it would be an option.
One final note on fuel economy. If you drive slower, you get better mileage. It is just a fact of physics. Wind resistance goes up hugely as speed increases. Here is a pretty funny article about "Prius Backlash" from the LA Times that says how the drivers in HOV lanes are starting to hate the Prius drivers because they "only drive 65" ('cause they are able to monitor their mileage and hate to drive faster when they see it dropping).
http://www.latimes.com/news/local/la-me-hybrids10apr10,0,4636957.story?coll=la-home-headlines
Pretty funny. Everybody complains now about the high cost of gasoline, but I still see pretty much everybody breaking the law by going 5-10 miles over the speed limit. Slowing down just 5 mph can save 10% on your fuel economy. If the government announced a new miracle technology to save 10% on every American's fuel economy, there would be this huge celebration! ...until people realized it might mean we'd all have to slow down by 5 mph.