How is OKC cheap? They gave 2 max contracts to their 2 best players, KD and Westbrook.
They also gave a deal to Ibaka for $48 million dollars. And they couldn't offer Harden the max $60 million dollar limit, so they gave him a generous $55 million dollar offer and he refused.
You can't just hand out max contracts to everbody. That's why there is a salary cap.
Oh and fyi, Harden shot 38% and had 14 PPG as a starter. That's not max contract worthy..