The Maryland State Gaming Commission has to post the money won by casinos each month. They break it up into categories, and there is one category for "non banked games" which basically means poker (other tables games you play against a "bank"). So it is easy to see how much each casino made from poker each month. All the stats are available at
http://gaming.mdlottery.com/financial-reporting/
I only see the last 10 months posted. The previous ones are probably still on the web site, but I was mostly interested in seeing how MGM changes the poker scene when it opened in December 2016. Here are the stats I gathered from that web site (no guarantee I didn't make any mistakes copying/pasting, but it should be mostly correct):
2017 Non-banked games revenue
April 2017
MGM $1,735,761.00
Horseshoe $513,986.00
Maryland Live $1,415,476.00
Hollywood Perryville $92,942.00
March 2017
MGM $1,768,988.00
Horseshoe $469,799.00
Maryland Live $1,524,579.00
Hollywood Perryville $113,468.00
February 2017
MGM $1,619,441.01
Horseshoe $401,830.00
Maryland Live $1,567,083.00
Hollywood Perryville $95,158.00
January 2017
MGM $1,806,791.00
Horseshoe $440,372.00
Maryland Live $1,581,390.00
Hollywood Perryville $93,407.00
December 2016
MGM $1,412,021.00
Horseshoe $503,047.00
Maryland Live $1,871,491.00
Hollywood Perryville $88,755.00
November 2016
Horseshoe $630,103.00
Maryland Live $1,965,323.00
Hollywood Perryville $77,837.00
October 2016
Horseshoe $650,367.00
Maryland Live $2,022,016.00
Hollywood Perryville $84,367.00
September 2016
Horseshoe $617,758.00
Maryland Live $2,037,388.00
Hollywood Perryville $83,880.00
August 2016
Horseshoe $659,907.25
Maryland Live $2,050,122.00
Hollywood Perryville $78,376.00
July 2016
Horseshoe $703,327.00
Maryland Live $2,298,935.00
Hollywood Perryville $89,891.00