This is the only site that I could find that gave anything near any actual figures.

GP

http://www.google.com/search?hl=en&rls=com.microsoft:en-US&rlz=1I7SKPB_en&&sa=X&ei=jDRxTPzjEcHflgfin7zvDQ&ved=0CCEQBSgA&q=effects+of+casinos+in+vicksburg+battlefield&spell=1

"Our estimates of ancillary spending are very generous considering the experiences with
the casinos in Atlantic City and Vicksburg MS. In Atlantic City there are fewer
independent restaurants now than before casinos arrived. In Vicksburg there was a steep
decline in attendance at the Civil War battlefield (25% compared to Gettysburg) after the
casinos opened. The same happened at Harpers Ferry (25% compared to Gettyburg). See
Miller (2006) page 86 for a discussion on this. The reduction in visitors to nearby
historical sites illustrates that, given the time constraints on recreational activities, people
have to make a choice of what to do, and mainly they don’t do both historical tourism
(the core of Philadelphia’s tourism market) and gambling. The gamblers also prefer the
subsidized food at the casino to outside restaurants."