does anyone have a clue as to what a 40- 50 or 75 gallon gas water heater average costs to operate is per month????

Thats completely open ended...totally depends on useage.
could be $15 a month....could be $1500 with a high maintanence wife.
I just ask my customers to pull out last summers gas bill if they have a boiler...otherwise the most recent bill for comparison.