I don't really budget for my bike. I don't set limits on cost to run it. My commuter bike always gets what it needs. It's too important to neglect it.
Having said that, it doesn't mean that I splurge. When I was commuting 9 miles each way in Palm Springs, my commuter was a 35 year old American Eagle/Nishiki 10 speed. My ex calculated that it cost me $6.00 per month to operate. It was wonderfully dependable.
My current commuter is a 17 year old Specialized Hard Rock with street slicks, fenders, rack and lights. It gets parts replaced as needed. Though I have fancied it up with brighter lights and toe clips. Once a bike meets my commuting needs, I don't need to play a lot with it.
My big splurge has been a Bacchetta Giro 20. Sometimes I commute on it. But its real reason was to do longer rides on weekends. I also have an even more frivilous bike, a Giant Stiletto chopper that is mostly used to ride in parades. The Hard Rock paid for them both in the money it has saved me as compared to any other transportation.
