Even if you suck at pvp you get 2 marks every round, rounds take 8-12 mins. After 3 rounds you can get between 6-9 marks. Farming for materials is firstly boring and takes like 30 minutes to get roughly 20-40 if you know all the spots. 40 mins give or take to get 50 materials giving 5 marks. Terrible way to gain crucible marks. I personally never even used the exchange because of this reason