Retrofitting is expensive. Most affordable S&S coupled frame and the only one mass produced (I believe) is the Surly Traveler's Check, identical to the Cross Check cyclocross bike. I own one and love it for road or dirt road touring. Surly is currently changing this over to an S&S coupled Long Haul Trucker with 26" wheels because they pack in the box better than the 700c wheels and are easier to service and/or replace in more remote areas. Price is around $600 wholesale including the coupled frame and matching fork thru Quality Bike Parts, most any store can order them for you. I like the hardshell s&s case for protection but you do have to think about it a bit -- I have done loop tours where I fly in somewhere, stay at a hotel near the airport and build the bike, store the box and anything else, then stay in the same hotel on the last night weeks or months later and pack up bike and leave. Most hotels will not charge you to store the box. I have also done point to point tours and sent the box ahead to a friend or a hotel who will store for me.