We should not forget that $1billion was the fund initially allocated for RRoD. But the failure rate turned out to be up to 3 times the initially estimated one on the first mobo version, it took more than one mobo redesign to solve the problem and only XB360S brought the failure rate down to industry standard, so also the total number of consoles affected was around 3 times what initially estimated, although successive redesigns, while not solving, lowered the failure rate. Finally, MS made things worse sending refurbished units, this caused multiple faiures to many users and so furtherly increasing the costs until the latest mobos became available, finally lowering to acceptable values the refurbished and replaced units' failure rate.
We won't ever know how much RRoD has really cost MS unless MS itself tells us, anyway, as its entertainment division includes many other products besides consoles, and MS also accounts for every loss and cost it can in a separate balance sheet item that includes some unpredictable costs and loss for every division and also costs for R&D that can be applied to more than one division. But given the little we know, it must be more than $3billion.
