My pH board is about 10 bucks for me to make at single board costs, and about 30 more for a decent probe. My ppm board is similar although I cant seem to find a good cheap COTS probe so I am using one I made. As of right now I have an arduino controlling lights, pumps, fans and it is monitoring pH, ppm, 4 temp probes and 1 humidity sensor. I am finishing up a simple host application which will have some graphing of recent ph and ppm and icons for accessory states, and will also allow for remote control and saving data to a db/external source.