Depends on the rewards Amex gives you. I assume that since you are asking this question on a frequent flyer miles forum, you would gather miles for the Amex rewards. My site below tells you how to value those miles for the use you would give them. Click on Free Frequent Flyer Miles in the left menu, then Value of Miles in the topo menu.

But why would you pay $40 for an Amex card? You can get one for free (for a year) plus a bonus of 10000 Delta Airlines miles for free. Click on Credit Cards in the top menu.

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The Personal Travel Experience of Gary Steiger
http://home.earthlink.net/~pgary/TravelFrame.html