Not just battery cells sourced from the GF, but once the Model 3 is in production their costs for most of the raw materials for all their cars will be going down. Where anything is shared between the S/X and the Model 3, their prices will go down because they will now be buying in 10X the volume. There may be some sub-assemblies bought from third parties that are shared, but what will be clearly shared are basic things like sheet aluminum, fasteners, and wire.
I would not be surprised if they are looking at every part of the S/X right now to see if changes could be made to share parts with the Model 3. Most of those are parts that would never be seen by the consumer, but every sub-assembly that could be shared with the Model 3 is a cost saving for the Models S and X.
The Model S and X also have very high profit margins. They could cut into those a little and turn a greater profit per car line. The pent up demand for the Models S and X are huge and one of their greatest criticism is they aren't luxurious enough to justify the price tag of a premium European sedan, though they are on par with less expensive luxury cars in the $60-$80K range. Between cost savings switching cell sourcing to Nevada, economizing parts with the Model 3, and shaving a little off the margin (about $5K), they might be able to lower the price of the car $10K-$20K. When they reintroduced the Model S 60, sales of the entry level car went up as those who wanted a Model S, but couldn't quite afford one jumped on board. If the prices across the board were $20K lower, the cars would be affordable to a larger segment of the population and they would sell more, especially as news of the Model 3 gets out there and there are a lot of people who get excited about it but don't want to wait a year and have the income to afford a car costing around $60K.
Right now Tesla needs to milk as much profit as possible out of the S/X line to support the push to get the Model 3 out the door. Once the Model 3 is selling, it will turn from an expense to an income source and they can afford to cut the margin on the S/X a little and still make money.
