I am trying to get a sample letter of interest that include fee
schedule for a legal witness expert on bridge structures to present to
Caltrans (CA Dept of Transportation)

You could look at a site like http://www.rudydegger.com/Fee.htm ,
adapt it to reflect what you charge, transform it to letter form, and
there you go.... If you want a letter with more "contract"-type
provisions, see also http://www.gillengineering.com/feecharg.htm .