@nexus - In my mind, you're overcomplicating it, and talking about some things not really related to the issue at hand. But, you definitely touch on the right points.
It's not this:
It IS this:
What we're talking about is this. Take two IDENTICAL arrows. Same everything, except on one arrow you use a 100gr field point, on the second arrow you screw off the field point and use a 100gr fixed blade broadhead. Then, you shoot them out of the same bow using a shooting machine.
I don't want to say drag from the vanes is irrelevant, it's just that it's the same between the two arrows, so irrelevant in talking about the difference in drop between those two arrows.
What is relevant is that the fixed blade broadhead will have higher drag coefficient, will slow the arrow faster, will have a lower average speed to target, and will drop more than the field point.
If you tune them to hit the same at close range, the broadhead will be low at long range. If you tune them to hit the same at long range (i.e. planing up), they'll be high at short / mid ranges.
The magnitude of how much difference you see and at what range would be due to a whole number of factors. Think about drag coefficient, drag in a laminar vs. turbulent flow (linear vs. exponential), momentum of the projectile, speed of the projectile, etc, etc.
And at the end of the day, the difference is going to be pretty damn small at short ranges to the point it doesn't matter for most people