It should drop to the best part of nothing.
Even if the batteries are good - losing 2v might well be enough to fail to get the engine turning.
With ignition off, use your second favourite big screwdriver (second, as it will have big nick in it soon!) to bridge the big two terminals on the solenoid.
Starter should spin but not engaged with the engine. Future progress then depends on: -
Still fails to spin, starter bugg**** (but isn't it, or at least the solenoid, newish)
Ditto, trouble in the supply. Engine Isolator??
Big fat spark and attempts to turn the engine, pinion stuck in engaged mode
Starter spins, so far so good, now contortion yourself to do the same after turning the key to pull the solenoid in!
Good luck!