If it were me, I wouldn't spend more than 2 times whatever the speakers cost on speaker cables. The only exception would be if you are going to bi wire externally (2 separate runs). Use the same pricing structure I list above about not spending more than 2x what the speakers cost, and just get 2 pairs.
To clarify: Speakers cost $1000, single run non-biwire no more than $2000, double biwire, single run cable x2 - $4000.