Well, it depends if you buy it from a store or used on ebay or from a private dealer. If you go to a store, it will be more expensive because of the middle man. I'd expect to pay at least a hundred dollars.
The price of the ring will depend on the style as well as the weight of the platinum ring that you buy, but as a quick reference, a platinum ring will cost about twice as much as an 18k gold ring.
