I run an eyeballs/isp network for about ~50,000 subscribers, and I see about 1:10 ratio at peak time.  Last night ~4.5 gbps out, ~45 gbps in.  But, I do have local caching of 4 big name cdn cache providers, so that might alter the 1:10 ratio I see on my actual inet links (which do not include the local cdn traffic)

…take Netflix for instance… I see on my local nfx cdn links, 1:100 ratio of in:out.  20 gbps inbound and .2 gbps outbound  (during that same timeframe as aforementioned actual inet links)

Numbers based on 21:00 CDT last night.

-Aaron