For the first 7 years I commuted, it cost me next to nothing to commute...I think the only thing I spent money on was about $20 for a light...I was commuting on my road bike and commuting was just another use for my existing equipment (so yeah, I did spend a few thousand bucks on those bikes, but I didn't buy them for commuting).

This year, I finally got a dedicated commuter (Swobo Otis ~$700), rack (~$35), panniers (2 different sets on sale, ~$70 total), new lock (~$30), new lights (~$35), and rain pants (~$30). My recent commuting related purchases have been around $1000.

I generally don't rationalize biking based on how much money I'll save on gas...I started commuting when gas was about $1/gallon :-). I bike commute because: I enjoy it, it relaxes me, it is good exercise, it is good for the environment, I live close enough that I would feel bad driving, and yes, it saves me some money (parking costs are about $200 a year, and at the current prices, I save about $15/week in gas...but this is just an added benefit).