http://dmn.delta.com/ace_offers/julymr/"For every 50,000 points you transfer, you'll be rewarded with a certificate for 20,000 Membership Rewards points–that's a 40% bonus back to you."
Because you get 20k MR back, it really only costs you 30k points.
So you get 50k SkyMiles for 30k MR points
That's 1.67 miles per point, a 67% bonus
Downside is its only in 50k increments and possibly you may not get 20k MR points back for some time