I have a 2016 Model X with 200-mile range, what is the cost to upgrade the batteries to have closer to a 300-mile range?

You don't upgrade. Trade the car in for a higher capacity model.

60 can be software upgraded to 75 ONLY.

Only upgrade you can do is on a small batch of the 60 because the reality is they are 75 you can do that from your touchscreen and the battery refreshes over night other models you have to trade in

Trade it in.

I didn't think you could upgrade it after purchase.