That depends on a lot of things...bear in mind we are not in the business of paying people to drive our cars, we prefer to make enough to sustain life and buy groceries if possible. That throws the $15k and $10k numbers out the window, as there isn't that much markup in a Cayman. Would I give you $5k off a PDK car that is on my lot? That's likely, and depending on the list price and how much is left in dealer margin I may even be willing to look around for you. The issue here is we all need to manage our inventory levels, and most of the time if I ask another dealer to trade with me they just want me to buy their car and I keep all of mine. Since that doesn't reduce my flooring expense the only benefit is whatever dealer margin is in the deal. At the level of discount you want there aren't many reasons to do business.
No one likes to be taken advantage of...neither the buyer nor the seller. If you have an interest in a win-win scenario I'd love to assist you...send me a PM if you wish.