There's a guy at work who loves to bike, but won't commute. When he checks out my bike on days I commute, I rib him a little about how his commute is half as long as mine and he should try riding to work. It's all in good fun.

I don't ever mention gas money, though.
Excercise, yes. Health, yes. Fun, yes. Money, NO. Not only do I consider it tacky, but it's also a poor argument. Some people have too many errands to do with too much ground to cover for it to be practical for them. I can't say it often enough...if your only motivation to bike is to save gas money, you won't stick with it.