For AMP, we've only gone with AdSense and haven't compared to any other ad network yet (would love to try out Sovrn, though, to compare). The CPMs for our AMP ad units on AdSense are:
320x50 BottomFloat = £0.77
300x250 Intext1 = £0.58
300x250 Intext2 = £0.33 [*]
300x250 Intext3 = £0.16 [*]
[*] CPMs are adjusted for reduced impressions of ads further down the page, as in AMP ads only get loaded when they're viewed
That equates to a pageview eCPM of £1.84, which is actually higher than what we've managed on non-AMP mobile pages with header bidding. That's quite surprising because on any non-AMP page, we've seen terrible performance of AdSense -- we even A/B tested AdSense vs Prebid.js on our mobile pages, and Prebid.js did much better. But not better than AMP-AdSense.
My comment about Sovrn's performance on mobile comes from our header bidding setup for non-AMP mobile pages. In our header bidding, Sovrn doesn't make us much money on mobile compared to other ad networks.