Routinely $ 70 and now $ 45+tax.
Have been monitoring it for a while, up to yesterday. The prices was firmly $ 70 everywhere for PC. Xbox version has been much cheaper. Note the Vessel of Hatred review score is not great compared to base so set your expectation properly.
Other version are also on sales:
Diablo® IV: Vessel of Hatred™ – Ultimate Edition $ 67
Diablo® IV: Vessel of Hatred™ – Deluxe Edition $ 45
Diablo® IV: Vessel of Hatred™ – Standard Edition $ 30