Exactly, I wouldn't be surprised if a driver's paycheck is a substantial part of the total cost, so you might end up needing two to four trucks to haul the same payload due to extra weight of the batteries (and there's an 80k combined weight limit afaik). At $25/hour average (googled) trucker pay that 200 mile trip taking 3-4 hours would cost > $75 per driver. Fuel savings are calculated to be $140, every additional driver costs at least $75.
That's maybe the best use-case for FSD I've yet to see.
The average weight of a reasonably new semi is around 20k pounds without fuel which leaves 60k for cargo. Every pound the tesla weighs above 20k is a pound less cargo.
Another consideration is the fact that tesla trucks won't be paying for huge amount of road wear they cause. The national average of state/federal tax on a gallon of diesel is around 65 cents. As things stand now an electric semi (just like electric cars) will be getting a free ride.
That's maybe the best use-case for FSD I've yet to see.