As you can see from the responses so far, people tour for all sorts of reasons. You can't really determine how much it'll cost until you determine what you expect out of the trip.

Some people get a big charge out of touring with the least, lightest, or most minimal gear possible. Others attempt extreme frugality. At another end of the spectrum you have the credit card tourers who don't even need to carry a tent or a stove. Most people figure out what's important for themselves and design a tour to meet that need.

My fondest memories of my bike tours are always of the people I've met along the way. Ironically, I've found I meet and talk to more people while I'm touring solo, so that's the way I choose to go. In order to have a chance to relax and talk to people, I tend toward organized campgrounds, local diner breakfasts, and an occasional motel room tossed in to escape weather, mosquitoes, or lack of camping options. And, ice cream! I never pass an ice cream stand on tour without stopping. Those of us that struggle a bit with our weight get precious few opportunities to eat all the diner food and ice cream we want and still lose weight while burning massive calories on a bike tour. That's the way I plan my tours and it works well for me. I figure on spending $40-$50 (US) per day. It fits my budget and, hey, I'm on vacation! I want to enjoy it.

Figure out what floats your boat and your budget limits. Find a way to meet both of those constraints, even if it means shortening your trip. Don't be one of those miserable souls that suffers through an entire tour just to say you did it. Plan something you can enjoy and have the time of your life.