Is it worth it?

So AMEX is running a promo that with each over $100.00 purchase on Delta you get 5,000 membership rewards. So I was thinking of buying 4,000 air miles which have a 100% bonus now for $140.00.

So is $140.00 dollars worth 8000 delta air miles plus 5,000 MR's?

Thanks for your time,

Jesse