I know there are many people who play it seriously and spend tons of money to get to maximum ranking, however for myself since I am one of the originator generations who played the Diablo series I just play for the joy of the unique gameplay of Diablo... brings back good memories from the past.
Money wise, this game is designed to pay to win... So I can spend up to 100 dollars for the joy of being entertained however I dont think I will overspend more than that... Its a trend that will come and go... unless your die hard fan of this I just like to play with family and friends time to time.
