We don't know if the SR Model 3 will be identical to the LR other than battery, but it is so for the MR other than AWDCreated a battery price table for some perspective:
MR Price, kWh: 46k, 66 kWh
LR Price, kWh: 53k, 78 kWh
$7k for 12 kWh + AWD. I think Tesla tries to charge $3k - $4k for the AWD so the extra battery cost is
$333/kwh - $250/kWh