They're buying off-the-shelf batteries from the same suppliers that build batteries for the rest of the portable electronics industry. Since batteries are a resource intensive product (they're made from commodity materials that must be mined and processed), there is always going to be a fixed cost associated with their production. Here's a free hint: more electric cars being sold will only put more demand on battery manufacturers, and I don't have to explain how supply and demand works.
You are dead-on with with the reflection on the maturity of electric vehicles. They've been around a LONG time.
But regarding battery manufacturing, you may have missed the recent news about Tesla's plans for building the world's largest battery factory this year - it seems that Musk has anticipated your concern: