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You pay the current price that gets locked in. So far, Tesla's policy has been to allow you to keep the old price if they raise prices in the interim period. However, if they drop prices, they will honor the reduced price (you may need to ask for this). It's a pretty generous policy. Be warned that if you ever make any change to an existing order (change color/wheels, etc.) then you no longer keep your old price and will end up switching to the new current price.