When it comes to selling diamonds you certainly can not compare it to any other commodity that you sell as each and every piece is different and its pricing varies depending on the demand and the 4 c's. There is always a lot of negotiating and every single dealer/jeweler will make you an entirely different offer.
Hi Vincent, on a $25k ring, I think the jeweller would be EXPECTING you to try to negotiate a price. It sounds as though its a bricks and mortar jewellers, not online, therefore, they will be working on higher margins than an online retailer. I'd suggest you could squeeze him to $20k or so.
It is a "franchise / shop" jeweler quite reputable here in Australia hence why i was a bit reluctant to haggle on the price. The price for the "diamond" online seems to be around 18k so i think 20k is a reasonable price.