The value realised from reward points depends on several factors, including what you purchased, the type of card you have, and what you redeem them on. The best value comes from flight upgrades, followed by flights, followed by store purchases.
For example, for each $1000 you spend, you typically earn 2000 Amex reward points*. When transferred to, say, the Qantas frequent flyer program, you get 1000 points, which if redeemed on a flight upgrade can be worth up to $64 or 6.4% of the original spend**. That’s at the top end, but on average, you can expect to realise around 2-4% of your original spend when redeeming points.
* The rate varies between 1 and 4 points per dollar depending on the purchase and the American Express card used.