Firewood gets sold by the cord, the face cord, the load, the cubic metre and the truckful, which makes comparing two prices nearly impossible. Measure the stack, get every unit at once, and see what you are really paying per cord.
Notes
What a cord actually is
A full cord is 128 cubic feet of stacked wood — about 3.62 cubic metres. It is a volume of stack, not of solid wood: the air between the pieces counts.
Face cord is not a real unit
A face cord is one stack 8 feet long and 4 feet high, but only as deep as the pieces are long. Sold as 16 inch pieces that is a third of a cord; as 24 inch pieces it is half. Two sellers quoting face cords can be selling wildly different amounts.
Stack it properly before measuring
A loosely thrown pile measures far bigger than the same wood stacked. Measure the stack, not the heap, or you will pay for air.
What this does not do
It measures volume, not energy. A cord of oak holds roughly twice the heat of a cord of poplar, and seasoning matters as much as species. Volume alone never tells you what a load is worth.