I did do some figures when they first implemented it but I threw away the Postit note I scrawled them on. It will change all the time with XMR exchange rate anyway. But the way I did it was to, after not mining overnight, make a note of the total mining income number and then run it for 1 hour. Then after stopping it, you need to wait at least 5 minutes for the next BTC credit to appear then just divide the increase in income by how many hashed you did in an hour.
Thanks, I plan to try and get some ballpark data on it, but I'll have to shut down the browser miners and wait for stats to sync for awhile, and that's not something I want to do right now. I've got several running and might as well wait until a prime stopping point when I can shut them all down together and then let the stats sync up. Then I'll get data points from a couple concurrent browser miners at the same time that will have different hash rates.