The English word "cheap" is also descended from the same Proto-Germanic word that Dutch "koop" is descended from. English even has the phrase "a good cheap" to mean a good deal or a bargain, although it's not very common nowadays. The word "cheap" used to mean "buy", but in modern times it is simply used as a shortened form of the phrase "good cheap".
Here's a line from Shakespeare that illustrates the usage of both "good cheap" and "dear" to mean "inexpensive" and "expensive", both usages still being preserved in the modern Dutch "goedkoop" and "duur":
"The sack that thou hast drunk me would have bought me lights as good cheap at the dearest chandler's in Europe."
Are equivalent distinctions made in Dutch between "inexpensive" and "cheap" as there are in English? That is "cheap" carries the additional connotation of oft times being of only adequate or lower quality, whereas something that is "inexpensive" is fair valued, a "good buy", if you will.