Most sensors for these parameters use wire electrodes that measure the resistance in water to judge pH, dissolved oxygen, and ammonia. Sometimes, like in the case with DO, the metals used in these meters are expensive and very sensitive, so the probes are very pricey.
Another way of getting the same readings is with optical sensors. These rely on shining a light onto a photochemical patch that reacts and emits/reflects that light down into a light sensor. Not only are such sensor systems more affordable, but they have other advantages as well.