The cost per unit depends on the timeframe for the options contract. If the options contract is for a short term (ie they want to exercise the options while the delivery of the initial order is ongoing) then the options will have a similar cost per unit. If the options contract is for a longer term (ie they might decide they want more of them after the initial order is completed) then it will cost more and have a longer delivery time because the contract basically pays for that production line to stay open but idle in case they need to build more. If the contract expires without being exercised, then there's a fee that is paid for the time for keeping that option open, and no further vehicles are delivered under that contract.
To put it simply, the options contract prepays for the time on the production line and storage of any tooling needed to fabricate non-common parts and carbodies, so if they exercise the options early, the vehicles will cost around the same as the firm order and take a shorter amount of time. If they exercise the options later, it will take a longer lead time to source the materials needed to produce more, and account for other factors like inflation.
