The main thing not to lose track of here - this car is designed for manufacturing efficiency. The cost of adding different drivetrain options to an automated production line is staggering.In terms of costs it's not comparable to 15 kWh of added battery capacity. What's the cost of an extra TO-247 for example, about $1.50 I'm not so sure that kind of thing breaks the bank compared to the battery capacity which is three orders of magnitude greater cost.
If they have different inverters (and likely they will have three) it's mainly due to efficiency factors and not cost.