For example Best Flights, 4% AMEX surcharge. The points-versus-surcharge equation is rather complex! I'll be buying two fares to Europe soon with B.Flights, so I'll have to make a call as to whether the (over $100) surcharge with the AMEX is worth the roughly 6,000 points I'll gain if I use the Earth AMEX.
Although, by using rather convoluted and unscientific maths
if one were to compare this with a SYD-MEL red e-deal at $125, for a gain of 1,000 points, that would value those points at around 12.5 cents.
Using that logic, 6,000 points at 12.5 cents would be worth $750. So it seems like a reasonable deal, compared to half the points and half the surcharge on a Mastercard, and no points and no surcharge on bank transfer.
Disclaimer: this equation bears no resemblance to proven or existing facts..