When you transfer 2 miles for 2c, you get 1 mile free = 2 cpm. I / anyone can give you much better options right now.

Sign up for the DL bounceback promo - That doubles your miles until end August.
Change your Amex credit card to a DL Plat card - same number and line, different rewards.
Take a nice home equity line for 50k. Pay your Amex with it by check at once or by pushing from a Citi Acct.
As soon as it hits Amex - in a couple of days at most the credit available will increase by 50k.
Pay 50 k taxes with it - you can do it on the Amex more than the credit limit if you paid in advance.

Ask for the taxes back. 50k on Plat Amex - gives you 50+50+20 bonus
Fee for taxes = 2.4% = That means 120k miles for 1200$ = 1 cpm
NOT worth it for any miles at other times, but with the double promo and the spend bonus, just about doable.
The interest cost on a HEL is about 3% - if the IRS has your money for 8 weeks -
that will cost you about 0.5c per $ = 50k = 250$ Total cost 1450$ still 1.2 cpm, much better than 2 cpm.

In general DL miles are worth about 1.2 cpm MAX. Honestly. Paying any more is quite useless unless you actually have an award ticket and are ready to ticket and the miles have to be bought right away for a C award.
If they bring back the 100% promo, well worth doing the transfer game - not worth it with 50% bonus.
Did that transfer 100% quite well with US until they shut my sister's account for a few weeks!
That was 1 cpm for US miles. Best I got from transfers on DL was 1.1 cpm 30k miles for 300+ 30$ fee.