When you're referring to vegetables in general it is the uncountable groente (like fruit in both Dutch and English), when you're referring to some specific pieces or types of vegetable, you can use the plural groenten.
- de groenteman / groenteboer verkoopt groente = the grocer sells vegetables (general)
- prei en sla zijn groente = leek and lettuce are vegetables (emphasis on the general fact that they are vegetables, non specific)
- prei en sla zijn groenten = leek and lettuce are vegetables (emphasis that they are specific types of vegetables)
- we eten groente = we eat vegetables (general)
- leg jij deze groenten even in de koelkast = can you put these vegetables in the fridge? (specific pieces)
"Goedkope" is related to English "good cheap". Before cheap took on its modern meaning of inexpensive" it meant "to buy, to trade. Mediaeval London had a market in an area called Cheapside (which still exists under that name) and the surname Chapman indicates a trader. German, Dutch and other Germanic languages still retain verbs for buying and selling that have the k/p/f structure of kaufen, kopen. Why English changed exclusively to buy Anglo-Saxon bicgan and sell (AS sellan Old Norse seljan is unclear, but *cheap" dropped out of use centuries ago..
Now, where's that bargain beer?
English isn't that far off. The English word "cheap" used to mean the same thing as the Dutch word "kopen", and English also used the phrase "good cheap" in the same way Dutch uses "goedkope". But for some unknown reason, English preferred the word "buy", which is rarely found in other Germanic languages, and eventually English speakers stopped using the word "cheap" except in the phrase "good cheap", which got shortened to just "cheap" and replaced the old meaning of the word.
The reason isn't entirely unknown. English has been heavily influenced by Norse, Celtic, Frisian, and Latin, among others. English speaking traders would often use the terms used by the people they traded with, while scholars liked to use Latin, which has led to a number of word sets that are based on one language in one context and another language in another context, even in the same basic type of action, as well as a number of "duplicate" words for similar meanings that ended up showing various degrees of intensity, etc.