dnd 5e – How much would a weapon cost that grants advantage on damage rolls?

101-500 gp.

Ilmari Karonen analyzed the average damage increase for such weapons here. Here are the results:

Die Normal Advantage Difference
d4 2.5  3.125 +0.625
d6 3.5 4.4722 +0.9722
d8 4.5 5.8125 +1.3125
d10 5.5 7.15 +1.65
d12 6.5 8.4861 +1.9861
highest of two 2d6 rolls 7 8.37 +1.37

A +2 magic weapon guarantees a flat +2 increase to damage, so all of these weapons will yield an inferior damage improvement to a +2 magic weapon, without providing the bonus to hit provided by the magic weapon. +2 magic weapons are Rare magic items, and all of these would be strictly worse, so these weapons should value somewhere in the range of Uncommon magic items, which is 101-500 gp (see the “Magic Item Rarity” table in the DMG). From here, it makes sense to scale the value somewhere within this range according to the damage improvement given, possibly taking into account that the weapons are not actually magical, and so would not overcome resistance to nonmagical slashing, piercing, or bludgeoning.

Note, if a feature adds additional damage dice to an attack’s damage, such as the Rogue’s Sneak Attack, it should not be rolled at advantage, as the improvement can get out of hand.