I don't think so. I haven't done detailed costing for a while but last time I looked at the numbers the open source project was more expensive than maintaining legacy drivers would have been.
For now, I'd definitely believe it. Hopefully once the r600g + Evergreen stuff is up + running + stabilized that balance will shift a bit. Long term, I'm hoping that the open-source strategy ends up saving you some money as it will possibly help to convince other hardware vendors to follow suit.
That being said, given that you do code-sharing between windows/fglrx, it might not end up that way, unless you consider that you're doing code/cost sharing via Gallium and state trackers with the other Mesa/Gallium hardware drivers.