Doubling the size of my bowl of oats in the morning and eating a couple spoonfuls of peanut butter seemed to cover the extra calories that I was burning for only a few cents extra each day.
I found that cycling to work (11 miles each way) two or three times per week did not significantly interfere with my own weight training as long as I got lots of sleep. That said, I'm a 200 lb guy who was deadlifting 400 lbs at my max.
Most people like to eat junk food too much, are kinda out of shape, and can save a few bucks by bike commuting . . . for them, it's almost a no-brainer. If you're already running yourself ragged at the gym, will see no financial benefit to cycling, follow a structured diet, and don't like to ride your bike then don't do it.