George, I think the point is to show how much you are saving over operating a car for that same distance. If you only rode your bike, this number has a great amount of significance.
LBD, if you just want to calculate the cost of consumables, that makes my figures quite a bit lower.

Excluding my BMX bikes, the cost to purchase or build some of my recent rides have ranged from $279 to around $8k. Once you get past the purchase price, they are virtually free to operate.