I agree that legally speaking, the man signed a contract and therefore there is no legal obligation for CDPR to pay him anymore money.
That said, it is HIS world and it has been far more successful than he and I'm sure CDPR ever imagined, so while I wouldn't agree to the 12 million or whatever he's asking for, I don't think an out of court settlement would be unreasonable to maintain that relationship with the creator of the source material.
As an example the woman who created the Nike "Swoosh" logo was paid $35 for her work in 1971. Although (out of court) she was remunerated 10 years later with Nike stock of an undisclosed amount and I'm sure she's done alright since. A gesture following such success can go a long way