Supply and demand
I work in the auto insurance field and have paid out my share of total losses over the years. The funny thing is, that on most cars and trucks with a manual trans-book value says they should be worth less. Even on sports cars.
However, those of us that love to bang our own gears see an automatic as one step closer to the wheel chair and assisted living facility--In other words, they are fine for somebody else.
With that said, if I had a farily rare four speed in the model you are after, I might be tempted to jack the price up a tad. In the same respect, if I just had to have an 80-81 then I would possible expect to pay more for something that is a little more rare and==Exactly what I want.
I don't think you will see an automatic (no pun intended) increase across the board for a manual trans. It will all depend on how motivated the seller is, and what kind of condition the car is in.
I just realized, I am going to get flamed out for my automatic transmission comments.
Nothing personal---I have owned two Corvettes with autos and loved them almost as much. Plus, my 3500 pick up is an auto, and I would not have it any other way.
I think I am just digging myself deeper here.