I ran some numbers with a coworker (as it turns out largely repeating
both of our previous hybrid analyses) and I'm of the opinion that the scooter as a supplementary vehicle makes little economic sense given the current price of fuel.
I already own a car which gets approximately 27 mpg on my highway commute. In order to supplement this with a maxi-scooter, I'd need to spend at least $6300 for a scooter that gets ~60 mpg and can handle highway speeds. At $3/gal and neglecting service costs, it will take me approximately 100,000 miles to recoup that initial investment from the gas I would save. That is a lot of scooter riding considering you aren't going to use it in bad weather or extreme temperatures. Given my commute, that's probably 8 years of maxi-scooter ownership just to recoup my investment.
Now a better choice would be a small motorcycle like the Kawasaki EX250. Although ironically smaller in displacement and horsepower than many big scooters, the Ninjette performs as well because it weighs much less. It gets similar gas mileage, but only costs $3000 new. With half the initial cost, it will only take half as long to recouping the initial investment as well. I find 50,000 miles or 4 years riding a lot more plausible than 100,000 or 8 years.
But frankly neither are especially plausible. If I had $3000 right now I'd pump it into fixing up the house we just bought and buying some reloading gear to keep my shooting costs down. I'm betting those things will reap much larger benefits than a motorcycle will. Which probably will keep Amybear quite happy.
UPDATE: A commenter said that putting a lot of miles on bike is pretty unusual. 20,000 is a lot of riding. Frankly, I think it depends on what people are riding the bikes for. Most people use motorcycles like they use convertible roadsters, they keep them in the garage and take them out on pleasant weekends for drives down twisty roads. Which is why you can get both used Harleys or Miatas with relatively low mileage for their age.
I think you can put miles on a bike or scooter if you regularly commute long distances. Not as many miles as with a car in the same situation though. You aren't going to ride in the rain or snow or cold or boiling heat. So you're looking at putting a half to a quarter of your yearly miles on a bike. And that's at best.
My real concern with high mileage bikes above simple fiscal economy is two fold.
First, the engineering. We expect cars to go at least 100,000 miles these days and twice that for a "reliable" brand. No one expects that from motorcycles and the manufacturers design accordingly. Even 50k is a huge number of miles on a bike. And motorcycles seem to be a lot more maintenance intensive that cars. When was the last time you had to sync the carburetors your car? When was the last time you saw carburetors on a car? Adjust the valves? Lube the drive chain? Replace any number of fluids and seals on the drive train and suspension? This is routine owner maintenance on many bikes and much of it once was on a car, but not so much these days.
Second, the safety. Lots of miles means lots of time on a bike. Lots of time means coming into contact with lots of idiots. Since half of all motorcycle accidents are caused by the other guy, the more time you ride, the more likely you are to have an accident. And an accident on a bike is not the same as a fender bender on a car. Not by a longshot.
While the first means something to me, because my time isn't free, the second means enough to my wife that pontifications on motorcycling are about all I'll ever be doing.
