any metal used for for cutting whether ferrous or non-ferrous, is usually sold per its thickness. majority of the time it's just the weight.

what i do is i figure the weight of the metal per sqft. or per sqin. most suppliers usually have a handbook of some sort to tell you weight per its size dimension.
once i know the weight, i use the price payed for the full sheet,and divide it up. i will end with with the cost per pound per square foot or per square inch. once i have cut the item out, i will then weigh it again this will give me my production weight. this seems to be accurate for me to use. especially with price per item and shipping.