Guaifenesin has been tried but i am not sure of proven effectiveness and it is not free of caveats. at i glance i saw for instance you can't have salicylates in your system.

i don't know anything about the dosages involved, nor whether there are risks for special populations (diabetics, hypertensives etc)

not sure of cost.

here are some links for further reading...

Fibromyalgia Treatment Center / Guaifenesin Protocol
Dr. St. Amand’s method for diagnosis
(c) 2000-2009

the following article includes comments on the protocol above:

The Truths and Myths of the use of Guaifenesin for Fibromyalgia
by Mark London
Last modified: 12/05/2010 15:47:42