I use the ion specific probe to measure for calcium, magnesium and *cyanide*. Quite accurate for Ca/Mg. Cyanide meter useless for hobbyists LOL.
Meter costs a few hundred $. Do not quite remember exactly how much, bought too long liao.
IMO, waste of money to spend such items on meauring NH3. You are not supposed to have them in your tank in any appreciable amounts anyway. Will it help if you know you have 57ppm of NH3 +- 1 ppm in your tank, or if you just know you have 40-60ppm of NH3?