Food you buy on tour doesn't have to cost significantly more than food you buy at home. It's different of course if you get free meals at home.

The thing you really have to budget for is accommodation, although roughing it and/or using warmshowers.org or such help. The problem with roughing it during bike tours is rain. There's no way to get anything dry in a tent overnight if it's raining all the time (or even most of the time). You'll need a shelter with a heat source every 3-4 days in rainy weather, otherwise all your gear will gradually get wet, including your shoes, tent, sleeping bag and sleeping pad. If you plan on touring in dry and warm climate, this may not be a problem for you.

--J