From the horse's mouth: *User revenue will be split 70% to the developer and 30%...

GHFigs on Feb 17, 2011 | parent | context | favorite | on: The true cost of publishing on the Amazon Kindle

From the horse's mouth: User revenue will be split 70% to the developer and 30% to Amazon net of delivery fees of $0.15 / MB. -- http://kdk.amazon.com/

That page also indicates the bandwidth threshold under which Amazon pays is 100kb/user/month.