The main thing is to look at the design structure diagram of the billboard (the structure diagram puts everything in it, such as quantity\size, etc.)
If it is an ordinary billboard, even if it is square, it is Length * Width. (Suitable for angle iron advertising light boxes (light boxes are also a type of billboard.)
If it is made of sand steel, it should be calculated in cubic meters. As for the price, it depends on the material. That is to say , the total quantity of materials required for each square meter or area is the cost, plus labor costs, and finally the profit, which is the final square meter/cubic you asked about.
Remember, square is length*width (in meters)
Cube is length*section*height (in meters)
The price of materials depends on the market price in each place