Both Tri and I have been asked to build these for sale. Last time I checked with him, Tri wasn't willing.
I quoted the person $700-1000, depending on the the cost of materials (~$450-750) and a LOT of my time invested when I'm not at my full-time job. There are shops that do this retrofit, I'm not sure what they would charge. Here's a link to one:
http://www.sickhids.com/