Sir I wanted to know what if by-products are sold for less than their costs? Do we then add those those net costs to total joint costs and then apportion those joint costs?
Byproducts are not costed. Any revenue from the byproducts is subtracted from the joint costs before splitting the joint costs between the main products.
I explain this in my free lectures on joint products.
The lectures are a complete free course for Paper MA and cover everything needed to be able to pass the exam well.