Rihanna very rarely sells well and this is a big week with big names releasing music, I know Kid Rock is releasing a new cd and he's a much better draw in terms of record sales. Also Glee just released a christmas album as well, and those always sell.
I would say Rihanna will sell around 150-200k. I'm not sure how much Katy Perry sold but I think something around that mark would be a good guess for Rihanna.