I'm personally too far from those negotiations to offer any likely-pivotal insight (such as a concrete quantity), but my very rough understanding is that there's some critical volume beyond which a customer basically becomes "made" with the Xilinx/Altera sales channels via a financially significant design win, at which point sales engineers etc. all but have a blank check to do things like comp development boards, advance a tray of whatever device is relevant to the design, etc..
Basically, as George Carlin put it, "it's a big club, and you ain't in it".
I don't have exact numbers, but I'm pretty sure you can get significant discounts starting around 100 parts. So not much at all.
Another thing to note is you can already get parts for significant discounts in 1-off quantities through legit Chinese distributors like LCSC. For example, a XC7A35T-2FGG484I is 90$ on Digikey and 20$ at LCSC. I think a personalized deal for that part would be cheaper than 20$ though...
