Delta charges a penny per mile to transfer miles and a $30 transaction charge.
You can transfer up to 30,000 miles in a transaction for a $330 fee and Delta will give your friend a total of 60,000 miles.
You can rinse and repeat back and forth for up 150,000 miles transferred from an account and up to 300,000 miles that can be received in an account per year.
It’s a cost of 1.1 cents per mile, and more like 1.06-1.08 cents per mile after factoring in points (such as 706 Ultimate Rewards points on a Sapphire Preferred card, 330 Starpoints on a Starwood AMEX, or 660 Delta miles on a Delta AMEX) gained on the $330 credit card purchase.
Delta accounts must be at least 10 days old and have already earned at least 1 mile to be eligible for the transfer bonus.
It’s worth noting that Delta’s saver availability is generally quite poor compared to other airlines, you can’t redeem for true international first class experiences, and that their Skyteam partners are weak compared to other alliances. Some of their policies like the non-changeable/non-refundable award tickets within 72 hours of departure along with fuel surcharges on some partners like V Australia are just asinine. However at under 1.1 cents per miles there are definitely many scenarios, especially in business class, where it can make sense to “buy” the miles via transferring with the bonus.