Does drag under pay deflect cost?

Yes, you must pay the Deflect cost if you use Drag Under to target a unit that has the Deflect keyword.

According to Rule 809.1.c, the Deflect keyword imposes a mandatory additional cost on any spell or ability an opponent controls that "chooses" (targets) the permanent. Since Drag Under is a spell that requires you to choose a unit to kill, it triggers the Deflect cost requirement.

Key points:

• Targeting: Because Drag Under targets a specific unit, it is subject to Deflect.
• Mandatory Cost: If the unit you are targeting has Deflect, you must pay the additional Power cost specified by that unit's Deflect value to play Drag Under.
• Payment: As per Rule 809.1.c.1, the Power used to pay this Deflect cost can be of any Domain.