The gigafactory will more than double the worlds production of lithium-ion batteries, which will drive the cost down. This will allow Tesla to pack more batteries in the car for a similar price.
Quoted from http://waitbutwhy.com/2015/06/how-tesla-will-change-your-lif... :
Right now, Musk says Tesla could make their cars with a 500 mile range—they don’t do it because it would increase the cost of the car.