Once the decision is made to keep him, you have to keep him happy. I think the suns should have traded him "because the team is in such a bad spot", but I can certainly understand keeping him.
Taking less money would have been stupid on Booker's part and ridiculous to expect. He is building generational wealth that will impact his family long after he is gone.
I seriously doubt, when Ish sells the suns for 5 billion, he will say to the buyer, hey since you're such a great person I'll knock 500mil off.