I'll relate what happened to me to explain where I came up with that 10% figure.
I was laid off from a job May 31, 2007. My employer had paid 100% of the premium. The HR director told me that COBRA would cost me the premium of $502, plus 2% administrative charge, bringing it to $512 and change. When I contacted the insurer, (can't remember the name) I was told it would cost $552, including 10% admin cost. I relayed what my HR person had told me, and the answer was "That's wrong. I wish people wouldn't say that, it's not correct." So I paid the $552, and thought no more about it, except that I thought it doesn't cost any 10% extra to administer my account.
I'm going to go dig through my old papers and see if I still have any relevant records, and if I have sufficient records, I'm going to contact the Department of Labor.