Flattr, and Why It's Not: http://www.websnark.com/archives/2010/02/by-the-way-the.html
The answer is made in their terms:
"Revenue to Site Owners: Site Owners receive 90% of the contributed revenue (monies contributed by Flattr Users and distributed to the Flattr accounts of Site Owners). Flattr retains the remaining 10% as its fee. "
I think that's just about reasonable. Certainly any more than that and I'd feel it was a little unjust.