Depending on the seller, be prepared to pay at least 50% and as much as 200% over the appraised value of a diamond.
There are several reasons jewelry can be so expensive. There are the costs associated with the mining and extraction of the precious metals and gems that are used. There is a transportation overhead to consider and the expertise of craftsmen. The jewelry store, located in a high-rent mall perhaps, has to hire staff and, as they say, "keep the lights on."
When it comes right down to it, though, diamonds and other jewelry cost so much because -- simply put -- they are pretty.