What are some first-order drag approximations for airframe/propwash interactions?
There is surprisingly little literature on that. This answer by @xxavier lead me to a very interesting paper in which the drag of a Luscombe 8b with and without thrust was compared. A very thoughtful way was used to arrive at zero-thrust numbers, and the result showed that the additional drag from the propeller effects was about 30% of the total zero-lift drag without thrust.

In the paper, the prop efficiency is 81% while the overall efficiency drops to 62% due to slipstream losses.