"How much does this gold necklace cost?"
Translation:Hoeveel kost deze gouden ketting?
but a ketting is a chain, like the bike chain.....i don't get it anymore
Ketting is the general term for all chains but often used for necklace specifically, it depends on the context. Like bike in English is a general term but can be refered with to motorbikes or bicycles specifically.
If you say "ik heb een ketting voor haar gekocht" then people usually assume you bought her a necklace. Unless she is a bit weird and likes the more kinky stuff or you happen to know her bike broke down last week and needs a new chain.