Watermeters.com offers nice bronze units with 1pulse/gallon, see datasheet here. They are not cheap, but any water meter with any sort of accuracy at this port size (i.e. low restriction) is expensive. That's still not the world's best resolution but better than nothing (i.e. ~8lb / pulse).Depending on how sensitive your pumps are to dead-heading, the flow restriction is not an issue - review the pump curve. Thus, if speed is not an issue (i.e. you're OK with waiting 2x+ longer than usual) you may be able to use those cheapie sensors you referenced. I wouldn't use such hardware below the water line, however. A less expensive option (open-loop) is to put a current switch on the pump. Measure the length that the pump has been running, multiply by a known flow rate (and that has to be verified with the local head conditions!) and voila. Current switches are cheap, Current Magnetics is one company that sells them.
You can also use a flowmeter in parallel with a pipe. In theory the flow will be proportional to the diameter of the two pipes. In practice you have to make measurements and create a lookuptable for different flowrates. The form (angles) of the connection of the pipes will determine how much goes through each pipe, you might need to experiment with that too as you like to have the max nr of pulses when the pump is at 100%.Not trivial testing, but can be done (watermeasurements are preferably done on a hot day
A ratiometric approach may certainly work. The drill pump is a good idea also. You may be able to skip the hall effect sensor and use a infra-red system as well if the vanes are visible. Such a sensor might be easier to mount and more rugged.
So I think the sensor can measure quite a bit more than SeeedStudio say!