The valuation principle relies on using a competitive market price to value a cost or benefit. We cannot have two different competitive market prices for the same good; if there was a good simultaneously trading for two different prices, everyone would buy at the lower price and sell at a higher price (arbitrage). The flood of buy and sell would push the two prices together until the profit was eliminated.
Law of One Price: in competitive markets, the same good or securities must have the same price. More generally, securities that produce exactly the same cash flows must have the same price.