That devboard is using recycled chips 100 percent. Their cost is almost nothing.
The kintex-7 part in question can probably be bought in volume quantities for around $190. Think 100kEAU.
This kind of price break comes with volume and is common with many other kinds of silicon besides FPGAs. Some product lines have more pricing pressure than others. For example, very popular MCUs may not get as wide of a price break. Some manufacturers price more fairly to distributors, some allow very large discounts.
Basically, as George Carlin put it, "it's a big club, and you ain't in it".
I don't have exact numbers, but I'm pretty sure you can get significant discounts starting around 100 parts. So not much at all.
Another thing to note is you can already get parts for significant discounts in 1-off quantities through legit Chinese distributors like LCSC. For example, a XC7A35T-2FGG484I is 90$ on Digikey and 20$ at LCSC. I think a personalized deal for that part would be cheaper than 20$ though...
[1] https://www.aliexpress.us/item/3256806069467487.html
[2] https://www.digikey.com/en/products/detail/amd/XC7K325T-1FFG...