suppose you can get a toyota prius for $20k [hah!] suppose you can get an echo, which the prius is based on, for $10k. the echo btw has good room for 4 and weighs 800lbs less than the prius.
It of course makes less stinky stuff out the rear, which a lot of people choose to include in the cost.
now also suppose the echo gets 30mpg and the prius 45. i know, they claim 55 but autoweek's experience over several months is a very steady 42-43. [remember also that the hybrid won't help you at all on the highway]
I have a friend who consciensously drives with an eye on the feedback meter thing, and brakes ahead up to the point where the friction brakes come on, and in general pays attention to the battery:gasoline:regen crossover, and gets low to mid 50's MPGs. He has severely changed his driving habits to match the car. It's not an unreasonable thing; we change our habits between AMXs with built 401's and older Americans with 195.6's.
then there's the battery replacement cost. they don't have a good handle on it, because prius sales are so brisk they're rapidly sliding down the volume production curve, but they don't last forever and it won't be negligible. and the take-outs are hazardous waste like all rechargeable batteries.
on top of all this, toyota makes a perfectly good diesel echo which gets about 45mpg [us equiv] in the european cycle, and it's not nearly as expensive as the prius. you can see it at www.toyota.co.uk as the yaris. and it can be fueled on biodiesel, if you want to be 'green'. some folks on the net are claiming to make it for under $1/gal but i think that's assuming free waste fryer oil, and methanol made from petroleum. but if you don't worry about food-grade processing [also look up 'cellulosic ethanol'] i wouldn't be a bit surprised if $3/gal wasn't in the range - and biodiesel can be used anywhere #2 diesel is, including your furnace. it's even better for the machinery, lubricates better and burns cleaner - and with a better smell.
Honda's Civic GX (natural gas) car available sort of in Calif. is a big winner, the cleanest I.C. car ever rated. About $22k, but tax breaks make it better and apparently more breaks on the way.