What MPG does a scooter get
This depends mostly on the engine size. The smallest engines are typically 49cc and they can give up to 100 mpg, depending on what type of riding you do and how much you weigh. Obviously if you weight 300lbs and do mostly short "stop and go" trips you'll get a lower mileage than if you weight 100lbs and travel mostly on long trips at modest speeds. At the extreme, I've seen figures of 130-140 mpg for 50c scooters when the rider was trying their best to drive as economically as possible.
Yamaha Vino 125cc - manufacturer's estimate - 96mpg
125 and 150cc scooters aren't quite as frugal, but I've seen reports of up to 95mpg and I've personally seen just over 90mpg from my 150cc scooter when driving gently. With normal driving I still see over 80mpg.
Yamaha Tmax 500cc scooter - manufacturer estimate - 47 mpg
With the largest engine sizes (500cc) you should still get around 50 mpg from a scooter travelling at legal highway speeds. That's about the same as a Toyota Prius Hybrid, but the scooter will be a lot more fun!
There aren't really any hybrid scooters available at the moment but Piaggio is developing an HyS engine technology which is a “parallel” hybrid in which a combustion engine and an electric motor are mechanically and electronically linked and simultaneously supply power to the wheel. They claim figures of up to 60km/L (170mpg). Piaggio were hoping to introduce Hys scooters in 2008, but up to now I've seen no word on price (which will no doubt be high) or availability.
The Vectrix all electric scooter is very economical once you've purchased it ($11,850) but it has some drawbacks. It has a range of between 25 and 65 miles and a top speed of 60 mph, so long trips aren't an option. Recharging takes 3 hours and a replacement battery pack costs $3000, so while it's very green, it's not very practical except for short commutes.