For material costs, it wouldn't be cheaper. There are ancillary costs though. Which were high enough that Tesla felt it was cheaper to eat the loss of using a 60kWh pack than pay those costs for the few 40kWh cars ordered. So few were ordered that they cancelled the 40kWh trim. The losses of making the few 40/60 cars for the people that had already paid a reserve fee was the cost of having good customer relations. Offering larger battery packs isn't as easy as offering a larger engine. They have a greater space requirement. The S chassis was designed for a 80kWh. An owner with a 60kWh pack can drive up a Tesla swapping station and get an 80 kWh pack installed in under 2 minutes. The Leaf was designed for the pack size(24kWh?) it has now. To put more cells in would mean putting them in the cargo area. Which entails building a case with ventilation for them. The rear suspension might need to be reworked. To get near 40kWh size pack, Nissan would likely have to add a liquid cooling system for the entire pack. I think optional pack sizes will be offered for new plug in models in the future. The current generation were built around the largest pack they could offer for the price point they felt would sell the most cars. Offering smaller is simple. Offering larger is going to have to wait until the redesign.
It would be most interesting to see a priced to build Leaf @ 40 or even 60kWh. I just wished we had the chance to know.
My guess is the a 40kWh leaf would be around $45k and the 60kWh $60k. The larger the battery, the bigger percentage it becomes of the price. With the rising price, Nissan would have to upgrade the interior and such to reflect the price tag. The Leaf was designed during the time when the batteries were costly. The costs are coming down. So range will improve, and they might offer differing battery sizes.
Toyota has the financial reserves (in case you wern't aware of this) to take a loss on the PIP or other plug in. GM, is an entirely different story. DBCassidy