The first question I would have for you is, according to you, it is a 25 mile drive. Is it highway miles or city miles? average speed you would be going? Because If its city driving, then you can go with a 600cc or less, but if you are going to be on the highway, I would go with 750cc or bigger. A 600 or smaller would beat you up bad on the highway. I've owned bikes for the past 15 years, and The small ones are MURDER on the open highway. If economy is the big deal, and you don't care about image, look at the Vespas!
http://www.vespausa.com/
Not very flashy, but I have a friend with one, and he is only driving in the city and LOVES the gas he saves.