The reason Tesla needs the Gigafactory, as you may remember, is to supply packs for the company's Gen 3 car, the EV that is supposed to cost $35,000 and be able to go over 200 miles on a charge. To get down to that price point, the batteries need to be cheaper, and Musk has said that the Gigafactory should reduce the per-kWh cost of a pack by "more than 30 percent." Speaking at the annual shareholder conference this week, Musk confirmed that planning for the battery plant is "quite advanced" and that Tesla is meeting daily with Panasonic, the other partner in the deal. Panasonic was originally unsure that those kinds of cost reductions could be achieved, Musk said, "but I think they are now convinced they can." In fact, he said 30 percent is "probably conservative at this point."
30 percent cost decrease is "probably conservative at this point" – Elon Musk
The Gigafactory should be up and running by the "late 2016 timeframe," Musk said, right around the time the Gen 3 car will be ready for volume production. While there will still only be one Tesla Gigafactory to begin with, Musk said that Tesla might start the process in two or three states (as opposed to just one or two, as hinted earlier). "We're probably going to do two or maybe three states all the way to creating a foundation and completing the plans and getting approvals and everything," he said. Sounds like someone wants to be ready to build gigafactories #2 and #3 in a hurry if need be.
We've got more from the meeting, including videos of Musk's talk, here and here. You can read Tesla's original Gigafactory proposal here (PDF).