Self-supported tours and commercial tours serve different requirements. If you like doing your own thing and being independent, self-supported is great. If you'd rather have someone else take care of reserving the accommodations, working out the routes and relieving you of the need to navigate (some companies do this better than others), and/or you're a bit uncomfortable about being in a foreign country and unable to speak the language, then you might like a commercial tour. Of course it'll cost more than self-supported: you're paying for someone else's time and labor, as well as their overhead (taxes, insurance, rents, personnel, etc.), and some level of profit.

Still the price-per-day varies quite a lot. One tour company, Undiscovered Country Tours, has a page online comparing itself to other outfits offering similar tours: http://www.udctours.com/about/biketourcompanies.html. At $150-300/day, they charge a lot less than most of the "other guys". (Disclaimer: The owners of udctours are friends of mine; however, I have no financial interest in their tour business).

I also operate a tour company, Agile Compass cycling tours (http://www.agile-compass.com/), though my tours are in France and Italy. My costs are largely in euros and so naturally track the dollar-euro exchange rate. Currently, my price for 2009 is under $230/day, which is a lot less than most of the "other guys" -- without cutting corners.