Nah, having done (a little bit) of battery design stuff, there's no way that the cost of the battery is at all a consideration. For reference, when I'm working on a system that needs "expensive", high-performance batteries with the best possible performance, we pay around 35 cents per watt-hour.

The Macbook Pro is around 50 Wh IIRC, which means if you replaced it with the most expensive cells I use it'd cost $17. And although you do pay more for different cell form factors (typically a decent amount more for flat batteries) the enormous scale that Apple operates at means there's no way the batteries are going to be a price concern no matter their size.

And yet these new "fewer materials" macbooks are significantly more expensive. =)