Actually the salt processing starts out in the Salar (the second stop on the tour) where they stack the salt to partially dry it. It is then trucked into Colchani to be processed, i.e., dried and bagged. My overall tour included all entry fees, so I don't know if there is an entry fee for the processing demonstration. Just in case, I gave the guy who did the demonstration a small tip.
Both of my tours to the Salar de Uyuni (2003 and 2006) stopped at this gift shop next to the building where they give the salt processing demonstration. It has a small museum also.