If I pay $200 for a printer and they immediately send me back $150 as a rebate because of a special deal, then what is the printer worth? It is worth $50 because that is the actual net cost.

If I pay $2B for Zynga and then pay myself back $1.5B from its cash reserve then it has only cost me $0.5B to buy. Hence its net worth is $0.5B.