For gallery Prints consider this
Gallery will take 50 % you will be required in most cases to provide image and frame.
If you are selling a image in a 30 x40 frame for $1500
then you will recieve $750
minus the cost of print and frame lets be super conservative and say $300
therefore the gallery recieves $750 - they handle the rented space, marketing and bring the client list
you recieve $450 - supplying the framed pieces
If you do not like this arrangement of funds up the price of your work and do the math.