I got an email today advertising up to a 45% bonus on buying points, but when I clicked through it looks like a 50% bonus is the max instead. For reference, the normal "cash value" of points is around 2.67 cents/point (rising to 2.8 cents/point if you have an AGR card), and the effective cost at 50% is 2.51 cents (dropping to 2.45 cents/point if you use an AGR card).