NBA contracts are based on comps -- that is, comparative elements. When negotiating a new contract, a player's agent points at a similar player and says "my client is like this player, so they should get this much" or, perhaps more commonly, "this player is making this much, and my client is clearly better than them."
NBA front offices can fire back in a similar way; "we love this player, but they haven't done this thing yet, and there is a player who did that thing making so much. Our offer is therefore less than that." And around and around they go.