I know the rule of thumb is a $1.00 a gallon on glass tanks 4'
I disagree with that rule. The line is drawn at 48"x13" footprint tanks. For anything ~60 gallons and smaller, $0.50/gallon is a good price. The wider footprint tanks, then $1/gallon. Once you get to 6' lengths, perhaps slightly above $1/gallon.
What is a fair price per gallon for a acrylic tank it has a custom black background,its drilled with corner over flow minimal scratches its 6ft 125 gallons and is up and running.
So what would be a fair price to pay for it?
For anything less than an amazing display tank, $2/gallon is too much, but $1/gallon is too low. I'll echo Ken and say that I don't see a significant difference between glass and acrylic until you get to ~220 gallons or larger.