What is the drag / power lost to a safety flag?
If the pole is a flexible one, you can do the following:


Choose a windless day
Ride past a second person at a known speed
Let the second person take a picture while you pass
Repeat immediately in the opposite direction to capture the effect of the remaining wind.
With the bike still, pull the flag back, measuring the force somehow. You can use a spring scale, or just hang a bottle with different amounts of water on a string, connect that string to the pole, and run the string over pulley or a round, smooth piece of metal.
Take more pictures of how you are pulling the flag back.
Compare the pictures of the bend pole while riding with the pictures while pulling the flag back.


With this method, you get the force that the flag exerts on you while riding the given speed. Now simply multiply with your speed, and you get the power that's drained by the flag.

Note: Work with the correct units. 1kg weights 9.81N, and if you multiply the force in N with your speed in m/s, you get Nm/s = J/s = W. Just make sure that you convert the inputs to the multiplication correctly.