I believe you are automatically eligible for DFP as long as you have adsense account. Self managed DFP account earnings is sent through the normal adsense payment method. (In terms of dynamic allocation that is), DFP is in essence an "adserver" which puts together the "partners" or agencies that you have. With this you have the capability to do frequency cap or geo target etc.
I believe DFP's dynamic allocation taps into Adx inventory. So you probably do not need to use DFP while trying to traffic individual Adx supplier codes. You want to minimize loops while maximize revenue.
DFP is free for the first x amount of impression I forgot. I believe it's something in the range of 20 or 60 million impressions per month, something pretty high. I just remember this figure was hard to find in their support document. So I am not trying to dig that out right now.
It's charged by usage or the amount of traffic that it serves through the ad server. But you will need to net roughly mid 4 figure or even 5 figures monthly to start having to pay for the service. The final cost of DFP is roughly 3~5% of the earning for my setup. But that means with a small account, DFP should be completely free. It doesn't hurt to learn and try to use it, (it's free to try after all).
DFP has a learning curve however. It took me a while to wrap my head around it.
The crash in RPM continues post valentine's day for me.
[edited by: frankleeceo at 6:42 pm (utc) on Feb 25, 2018]