Right now, I live 2 miles from work and am deployed 5 months out of the year. I put 6k miles on my 4Runner in the last 15 months, heh. Even when I was commuting 16 miles each way, though, I was still only doing about 700-800 miles per month.
I had been considering a prius as a DD if my commute distance goes back up. However, even if I got up to 10,000 miles per year with a vehicle that got 55 mpg, it would take something like 20 years to make up the difference in saved fuel (saving about $1400/yr in gas after the extra insurance is taken out, amortized over a $25,000 car, plus interest).
Of course, if you buy a beater for $1000 and are lucky enough that it stays reliable for any extended period of time without maintenance (and for many, like me, getting stranded is unacceptable), it might pay for itself in a year or so (assuming an old $1000 car gets 35mpg versus 15 in the offroad toy). But, I would not want to be saddled with such a cheap car without a lot of preventative maintenance, which means either a lot of additional time or money (which are basically the same...).
Honestly, Id suggest anyone looking to buy a 2nd vehicle purely for fuel savings should crunch the numbers for their situation to see if it is really worth it. Of course, buying a fun car like a miata, or a practical car like a small minivan, has its own benefits. I have been seriously considering buying myself a motorcycle; of course it does offer some modest fuel savings over my 4Runner, but I also just kinda want another motorcycle :P .