What aren't you understanding? 1 battery alone gains me .2V. Then another battery with an alt gains me .4V. Since you seem to want to help out 1800 here, lets assume the third battery didn't help one bit. The alternator helped .4V then.
Price of the alternator, I believe it was around $450-550.
My batteries in the rear were Deka 9a31s, I got them for $100 a piece.
So assuming that the alternator gained the whole .4V, .4V for $450 or .4V for $200.
I can go outside tomorrow and make a video if you want me to. Oh, and here's even something I failed to mention, and surely you can understand this one. When you experience voltage drop, that's when your amplifier demand is dipping into your battery bank. When I'd turn my car off for the Street B and C burps, my starting voltage would be around 13.2V. When my car was on, my starting voltage was around 14.7V. Funny, 13.2V to 12.6V with two batteries, and then 14.7V to 13.0V. Seems like it's the batteries doing most of the work there.