There seems to be a bug with this achievement that WB has even addressed: "Sometimes, the Achievement for collecting 10 billion studs (Delved too Greedily.) doesn't unlock once 10 billion studs are collected. Also, try to work your way to getting the 10x first so that after you spend the 10,000,000, you can regain the studs quicker since you'll have the biggest individual multiplier. TIP: A way to get to 10 billion faster is to try to avoid buying characters and other red bricks beside the multipliers until you reach 10 billion. Something else that could probably help is the Attract Studs extra - C7FJ7B. 10x - located in Cirith Ungol (to the left near two Mithril pieces) Craft Item needed is the Mithril Squeak Sword.8x - located in Helm's Deep (to the left of the map stone from map view) Craft item needed is the Mithril Shield (located in The Path of the Dead to left of final area before fight) Red Brick costs 8,000,000.
6x - located in Edoras (below the statue from map view) Craft Item needed is the Mithril Top Hat (located in The Battle of Pelennor Fields after fight with The Witch King, blow up one of the Mithril LEGO Pieces) Red Brick costs 4,000,000.4x - in Amon Hen (first quest to the left of map statue) Craft Item needed is the Mithril Fishing Rod.Applying them all together will create an even greater multiplier: 3840x. There are 5 (2x, 4x, 6x, 8x & 10x) like all LEGO games. To help get to 10 billion faster, you need to look to buy Stud Multipliers.