I bought mine from a guy on the Brompton talk list for £40. He had bought the clamp and mech for himself and never got around to putting it on his bike. I believe Steve Parry sells the clamp alone for £40. Then there is the mech which is about £15. The chap threw in a second hand friction lever and I bought a cable for £2. Then there is the front chainwheel. If you own a Merc, since it is made in Taiwan, and its parts are inexpensive, you can smile broadly and part with a mere £4.50 for the 39 toothed variety. Other wise, you might pay about £30 for one from a European manufacturer.
My entire conversion cost me under £50. It has made the bike more versatile by allowing a 25% gear reduction at the flick of a lever. On the Brompton Talk list at Yahoo, someone asked me why I hadn't used the full capacity of the mech by going for a 32/53 combination. The answer is I didn't feel I needed that much, but also that the crankset won't take a chainwheel smaller than 39. Check out the photograph taken through the frame to see why. The mounting bolts are as near the edge of the chainwheel as could be done with a 39 tooth wheel. If you want to get into new cranks, you can go as low as 32/53 with the mech I used.
The modification works brilliantly. It is totally reliable. One might ask questions about the ability of the chaintensioner to wrap enough chain to keep the tension correct, but I have had no trouble and only once had the chain jump off since I did the mod. This could have been due to other causes than the tensioners ability to cope with a smaller front chainring. I use the alluminium upgrade tensioner made by the Merc people.
PS I believe that Merc is a re-badging name for the Taiwanise flamingobike company. They seem to make a variety of qualities of Brompton lookalike, one with a horrid pressed allu frame, and other better ones like mine which I think is pretty good. They have upgraded their chain tensioners to all alluminium ones which makes me think they are the source of Merc bikes.
http://www.flamingobike.com/products...p?pid=29&cid=3