Shop around for gear tooth counters, Allegro models were selling for less than $10 years ago.
How they work is make a magnetic field with a coil that has a linear Hall sensor in front of it. Gear tooth bends the field as it goes by and that gets detected.
A linear Hall sensor runs about 30 cents and a button magnet costs little, you might roll your own sensor and code for that to save on a bunch of circuitry in the pro model. AVRs have a voltage comparator for instance.