At Green Man Gaming, search for "Dawnguard" to find downloads of The Elder Scrolls V: Skyrim - Dawnguard DLC for Windows
for $19.99. Coupon code "ORDER-NTCOM-PLETE" slices it to $15
. That's a $5 drop and the lowest price we could find by $5.
Of note, this product requires a Steam account to activate.