DIY Flow Meter
If all you want is to compare relative flow rates (or that you have any!) in your own system, here is a low $ DIY system. This is an idea I've had a long time, and haven't had the opportunity to build.
In simple terms, put a propeller (small, hobby type) in the flow, and monitor it's RPM via IR emitter-detector pair.
The easiest way to do this is inside a reservoir, particularly those cylindrical ones. Find or build/modify a plastic hobby propeller with a diameter a little smaller than the rez' ID. Place it on an axle (stainless, brass, or plastic) between the inlet and outlet barbs. Use a light source and sensor to monitor the spinning blade. IR pairs work well. I'm sure someone here can provide an op-amp circuit that will effectively deliver the signal to a fan speed header.
Take this idea to an inline setup: use a small, clear tube, with end-caps and end-barbs.