Didds is right but think about jow much you need as imo you dont need much. Marks are good for vanguard armour whilst leveling up once you're 27 though its anout getting you the highest gear either from the Crota Raid or Iron Banner.
I also find there's only a few guns that are worth saving up for. As you continue to play you drop more and more (and nightfalls) legendary engrams that will start building up your weapon inventory. I've only bought a couple of guns for specific reasons (pvp auto rifle and armour piercing sniper). I can now use my marks to get tons of materials.