Last week I bought a Specialized Sirrus bike. I've ridden it to work twice this week, and expect it to be my standard transport to work. With gas alone at current prices, I will need to save about 250 gallons before I pay it off savings. My tank holds about 15 gallons, so that makes about 17.5 fill-ups. At my standard of a a fill-up roughly every two weeks, that makes it about year before I'll break even. That's assuming I ride about 35 weeks a year, but that's also assuming gas doesn't go up. I expect those two assumptions to prove equally false.
In addition to saving money, I get exercise on my way to work. Gmaps Pedometer says I ride about five miles a day. It doesn't that seem that far, but that may be because the scenery is still new. I expect I'll eventually buy an iPod to entertain myself on the ride.
The bike was the third most expensive purchase I've ever made, after computers and cars (in that order). But I expect over the next year it will turn out to be a better deal than both.