Red Orbs is the money system in DmC. You can spend them on items that can be bought before you start a mission or at Divinity Statues. Killing enemies, finding the Lost Souls and destroying environmental objects (lamp posts, gambling machines, red cocoons) will bring you Red Orbs. 50,000 should take between 3 and 4 playthroughs to earn. I'd suggest buying Health and Devil Trigger crosses first as they're expensive and are very helpful for the Dante Must Die difficulty (they are also needed for "Let's rock, baby!" and "You can't handle it" achievements).