According to Blizzard Reaper of Souls isn't doing so bad when you consider all the fuss surrounding the original game, raking in 2.7 million units sold worldwide over both digital and retail copies. It's only a fifth of what Diablo III managed to draw in on its first week, but still not bad all things considered. Given the stigma that many players still have over Diablo III's pre-Loot 2.0 days it may still take some time for the expansion to gain some traction.
Regardless of how disappointing the numbers may seem it's worth noting that this is only the first week of sales of PC/Mac only release. With a PS4 version coming sometime down the line and an Xbox One version rumored in the near future this probably isn't the last we'll hear of Diablo or its game-changing expansion. You just can't keep a good Lord of Terror down.