Check out my motorized bicycle. You can get the complete engine setup on Ebay for about $200, and get 200mpg. I had this engine on a tandem when I took the pictures, but then put it on a regular mountain bike. You could easily get 200mpg if you put it on a road bike.
I picked up a Kymco Like 200i scooter in March as my primary means of transportation. Despite it's name, it is a 163cc fuel injected scooter. The cash price was $3200 out the door but that was with a windshield added to it too.
At lower speeds, it gets GREAT fuel economy. I've seen as high as 70 mpg, but I have a VERY heavy hand and am at full throttle quit often, but not because I have to. This thing has EXCELLENT pick up from zero to 45 mph. After that it takes it a while to hit it's 60 mph top speed. But with the luggage box and the saddle bags that I added, it can hold quit a bit of stuff. I have ridden it 90+ miles non stop and had luggage with me for a 5 day trip. On the highway when you're at full throttle the fuel mileage drops down to 50 mpg.