The US government says 240mg:
(Scroll down to table 2.)
I was just working my way through nutritiondata looking for magnesium-rich primal foods. The best by weight seem to be nuts & seeds - which is quite handy, as they can be ground & added to all sorts of things, sweet and savoury. Pumpkin seeds came out top (and toasted pumpkin seeds are really moreish), followed by flaxseed, brazil nuts, sunflower seeds, sesame seeds, almonds, pine nuts, etc.