And 15% of a game the size of Diablo 3 to simply integrate with a payment system seems a bit of a high estimate. I've actually done that work myself as a developer and it's not particularly difficult.
I wouldn't be surprised if it were 10+ % of the project schedule with coding and testing. You are looking at possibly millions of people banging on the RMAH and trying to hack it etc. Acti-Blizz has to be VERY careful with $$$ transactions (legal issues). Thats great that you have developed before, but I doubt you have handled code for thousands of $$ transactions. I'm sure it's more then "it's not particularly difficult". Hell, even PvP didn't make it into release...
Given all the years this game was in development, I have to wonder how many times they started over. There's no way the game they released should have taken anywhere near that long.
"The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." -- Bertrand Russell (I think...)