in vertically integrated business, even 2 times difference in the price of some basic parts/materials may not matter if that is just a fraction of the total price of the resulting product/service. If you own the high margin exit point like installation of the solar systems, you may be more worried about quality and the performance of the parts, stability of supply, etc... - the factors which can significantly affect your downstream high-margin part of the pipeline.

Add to that that Musk is going to own battery component of the solar systems as well.