The voltage drop is on the wires above the starter solenoid, I have noticed the same corrosion issues you talk about, I actually replaced all 3 of the wire ends and cleaned up the spade terminals and dielectric greased up the whole thing.
I am guessing there is more then a starter relay in that box since it has 3 wires coming in on the top, 2 of which do not have power until you turn on the ignition, and I agree with you, .4V seems like a ton of drop without the sled running. I checked on the input of the solenoid, 12.3V and the 10 amp fuse has 12.3V on both sides. the red/white wire and blue/white wire only have 11.88V
I circled the wire
Guess I should go get a new starter solenoid.