I think the couplers alone cost about $250. And that is BEFORE any welding or mods that you do on the frame.
I'm also not sure if S&S will allow you to do a retrofit. I heard they're pretty strict on who they sell to, and for what purpose..
I was looking in to this recently. It's possible to do, and most places seem to charge around $400 for the job (parts & labor). The S&S website (www.sandsmachine.com) has a list somewhere telling you what framebuilders will retrofit the couplers and how long it will take them to do it.
From what I read, retrofitting the couplers is more difficult than building them in to a new frame as it's time consuming to make sure that everything still lines up as it should.
i recently brought a bike back from Copenhagen. the airline checked it as if they were checking a bag. i was allowed two pieces of luggage, and checked the bike as my second, and it was free. they only required that the handlebars and pedals were off. i taped cardboard around the frame and was able to wheel the bike on it's tires, through the airport.
it was a United flight in case anyone was wondering.
oh, and i'm doing this next time i travel to Europe, in case anyone was wondering <g>
I had a bike retrofitted with the couplers over the winter. Any bike fabricator that makes their own frames should be able to do it. The shop I went to said that they would only do steel frames (although I have seen Ti bikes with the couplers installed), and the total cost was less than $500US. It's more expensive if you opt for a full frame repaint instead of matching up colors for the areas that need to be repainted (after they get toasted by the brazing).
It's expensive, but not really considering how much they gouge you for when you check your bike on a US flight. 4 or 5 trips and it's paid for itself.