in the bay area 10% off is a good deal on an order. From stock on the lot I would think you could press a little higher. I find the best way to deal is in $ above the invoice price. I would think that something on the lot could be had for $500 - $1000 over invoice depending on the circumstances. Your percentage off MSRP will depend on options.
Stripped base Cayman at 15% off is $1000 under the invoice. That will not happen.