It will be interesting to see if this is more than release weekend enthusiasm. Here's the steamchart graph with the player numbers for civ V and VI for the past month:
http://steamcharts.com/cmp/289070,8930#1m
This weekend hit ~80k which is about half of Civ 6 vanilla's post launch peak of 160k. And about double the previous weekend's average player base of like 40k.
So that would indicated R&F moved between 25%-50% of the original civ 6 launch sales. I'd probably guess it was a little closer to the 50% side - maybe 40-45% - as I'd assume the regular players are more likely to buy it.
That seems like good numbers to me, but I dunno how well expansions usually sell though.