The number of credits gained by Nasir Meidan is based on the rez cost of the ice at the time that his ability resolves, so the Runner can install a Rook during approach with Self-modifying Code and Deep Red to increase the number of credits he will gain on encounter.
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Say I have Deep Red and am playing Nasir. Corp Rezes, I CMC a Rook, trigger to host on newly rezzed card. Does Nasir get +2 cash?