All up Luai is getting around 700K per year from that 850K deal. With the 1.2 million dollar deal, he is still getting at least 1 million per year (agents fees and after tax included). So a 300K difference at least. That's big money. And totally worth leaving for. He can get more TPAs as the main man than being at penrith where his importance is way lesser. Also, I don't think he will play origin again, so money from that won't come by staying at penrith or anywhere else.
You forgot agent % from your calculations.