According to news, prices will be $3,000-$5,000 lower for average residential installations, which translate to about 10-15% of overall cost. In the case of bigger installations, it could be up to 20% less.
The reason behind making solar installations more affordable is"vertical integration". We assume that several factors contributed to the reductions:
in-house production of solar panels with Panasonic, instead of purchasing it from external suppliers
integration of solar products into Tesla's retail stores and the increased efficiencies over the past year
closing more expensive sales channels like door-to-door sales
sales of solar in combination with Powerwall (the prices of Powerwall increased)