Welp. So, I did some maths, and turns out, after all taxes and rent and etc., I have 309055.52 Yen to spare. Per month. Take note, though, that I took all American averages, and at the very end, converted it to Yen.
I took income tax, rent for the apartment (I added an extra 500 USD here, just in case), and average month cost for groceries. I can't think of what other things to deduct from my monthly, but that 500 Yen that I picked up ain't much.