The size of the print is just one factor in determining cost involved.
The biggest factor is the time necessary to produce the desired effect in post processing.
I have separated that work into three categories:
OUT OF CAMERA: This is basically as the frame was shot, no cropping, just basic sharpening, tonal and color balance adjustments.
EXTRACTED: In addition to the basic adjustments, this photo has either had the car extracted from the original background and placed in a different setting to create a compelling image or the background manipulated to create the same effect.
COLLAGE/DISPLAY: This photo is made up of two or more images extracted from their background and arranged in a pleasing manner on a new background.
The most popular in this category is the “Low Sky Series” and the different sized “Show Boards”.
So the amount of time invested to create the work is the most important cost factor along with the price of the print.