They don't really vary THAT much in these circumstances. This is always the absolute least helpful answer that someone ALWAYS seems to HAVE to post.
Imagine you went to a jewelry appraiser and paid them a bunch of money to tell you "its worth only what someone is willing to pay".
In this instance, it doesn't even make sense as an answer, because OP asked if the price was reasonable, not "what is it worth" as an objective statement.
Jewelers tend to over estimate value when appraising. They give you an insured value, not a what they would pay if you wished to sell it value. Generally, their appraisals are worth less than "what someone is willing to pay for it". If they gave you that figure, you would be pissed. Diamonds are plentiful and worth far less as soon as you buy it. Gold can be melted and is a standard formula, but that number is generally way below what you paid for the item.