Get your bills paid, make sure enough food is in the fridge and put a few dollars into savings each pay day, be it $5 or $50. Anything left over is yours to do what ever the Hell you want. Anything. Whether it be fill your life with trading cards, over priced ugly assed toys or anything else; it becomes a justifiable reward when all that other RL shit is outta the way.
There's no arguments, then. Especially if you make it clear to her that she's just as entitled as you are to spending the excess funds on whatever you like. We work hard to keep our house afloat, it's only fair we get rewarded for our efforts.
-Windebieste.