Because IF, and this is a big if, the breeder of a crossbreed litter has researched the market, researched what is likely to be produced, used a dog and bitch that are good enough to be used for pure breeding and have had health checks and has then decided to breed something which appears to be wanted then why shouldnt they ask a price that reflects the work and money that has gone into producing a litter.
Surely better than that than 2 pure dogs being mated indiscriminately and selling pups for the 'going rate' for that breed regardless of the fact that they havent been produced or reared with any care or knowledge.