Been a vegetarian for 38 years and counting (started at 15). I disagree with those that say you don't need more calories. That would only be true if you're going to do something else just as intense if you don't ride.
I estimate my riding burns about 5000 calories per week. And my diet is around 2800 per day. So I guess my food budget is about 25% spent on bicycling. But that's not a cost. I'm committed to fitness and would do other very active things too (and do, but I do love to ride).
Neat to see so many veggie responders.