The cap is low. The grind for 150 marks (cost of an exotic) is as follows: Assuming the legendary mark rewards remain at 6 for strikes and 6 for a Crucible WIN, then you would have to grind out 25 strikes and/or win a minimum of 25 Crucible matches to afford an exotic. If you win 50% of your games in Crucible, that would be 30 Crucible matches. Daily heroics give 5 marks (currently) and you get a few more than that for the Nightfall and Weekly. You can also earn (about 4?) for a public event. Translation: the grind will be real and you'll need to go spend those marks when you accumulate enough.