Why do people sell cockapoo dogs for $300-$750, if there a mixed breed? ?

Well im interested into getting a cockapoo dog. I know its a mix, and people have told me technically its a mutt. Ive also heard you can go to a shelter and get it easily for around $70. But my question is, why do people sell them for like $400? I think there the cutest things ever, and I just wanted one.. I dont even care about papers or whatever. Is it true that people rip you off when they sell them for $300-$750?