usually 1,5 Boxes of each expansion : 1 box plus occasional boosters here and there. This year, with 7th coming on it will be 7 expansions. That is 70 USD/month right there, in average.
Then the usual FNM tourney fee, plus occasional parallel expenses, like sleeves, boxes, binders I'd say easilly 75-100 USD / month.
Still a lot of money, but I have a work, and this is my only hobby. I guess playing golf or tennis, or going in ski vacation wouldnt come as cheap ! And it is still less than my electricity bill....