You really do just have to see for yourself like everyone is saying. No one can tell you what it's going to cost. I guess a good estimate is that you can do $10/day in the US if you never get prepared food and don't mind eating the cheapest repetitive crap you can find day after day. It will often be the same, like Ramen noodles. Peanut butter is another solid option for it's calorie content(~2500+ cal. for ~$3 of name brand stuff). Works great on crackers or veggies.

I would Highly recommend doing a week or so tour, or at least getting out and doing 50 miles/day for a week and see how you eat. Some seem to be fine eating what seems like a normal days meals. Others like myself for some reason seem to suddenly become very hungry all the time on tour if we don't constantly stuff calories into our faces. I think I ended up ~4000+cal/day on my last tour and I could have eaten more, riding 50-60mi/day. I was constantly hungry if I didn't get at least 4000cal in, which sucked some fun out of touring before I got on the right track. It was a surprise to me since I'm usually under 2000cal/day in my daily life.

I'm glad to see others not diggin the oatmeal. Even the higher calorie flavored stuff is only ~130-150cal/packet. At least for me, that means I'm trying to stuff, like, 5+ packs into my stomach for breakfast. That's just too much to eat. I also found that I get hungry not far into the day if I eat oatmeal. When camping, it's kind of messy for cleanup compared to other options too. I started out touring trying to do oatmeal because I read about so many people on here eating it, but quickly decided it wasn't a good way to go to get in the calories I needed. Ramen for breakfast tends to keep me going better. Pop-Tarts(generic if cheaper) are a good, cheap, high calorie late-morning snack.