Surprised nobody has mentioned a Toyota Corolla yet. 3 years ago when diesel his $2 plus a gallon and I was commuting 70 miles a day I bought a '99 Corolla as a commuter car for $6k. I get 37mpg commuting with the Corolla vs appx. 17 with the CTD. Depending on the price of gas vs diesel the mimimum I saved during a given month was appx $125, and at todays price it is well over $200 less per month to drive the car. I don't know what RVaugn is smoking but I have never replaced tires on a car before at least 50-60,000 miles, at a cost of about 1/4 what truck tires cost. As far as insurance goes, that went up less than $20 per month (with full coverage), and one trick is to list your commuter car as your primary vehicle. The insurance on my truck dropped quite a bit by listing my Corolla as primary and CTD as recreational use. Other advantages - cost of all maintanence is far less for the Corolla (about 1/3 for an oil change) and I have about 35,000 less miles on my CTD than I would have at this stage otherwise. In the 3+ years I have owned the Corolla it has not been in the shop ONCE, nothing but routine mainenance (wish I could say the same about the CTD).
Now for the icing on the cake, the Corolla has a retail value now of about $5k, so it only dropped about $1k in the 3+ years I have owned it. So in real dollars it cost me about $26 a month to own this car, while saving anywhere from $125-$200 a month in fuel driving it. The reduced cost of maintenance pretty much covered the increased cost in insurance, and to state again, I have far less miles on my CTD and would be out of my powertrain warranty had I not put those commuting miles on the car.
My suggestion, a Toyota Corolla, Honda Civic, or Nissan Sentra, which are far more reliabel than a domestic "commuter". A diesel is harder to overcome with the increased cost for a diesel powered car. Anything worth looking at when I bought the Corolla (i. e. diesel Jetta) was at least $10k, for maybe 5 or so better MPG, on average.