As liudr suggested I would also say weighing the sugar and water is the simplest and most accurate idea. The size of the container and it eventual weight will determine what scales to purchase. A quick google for 'weighing scales with rs232' bought up several choices and looking at my first hit Adam CPWplus P, M & L Weighing Scales seems it's doable with an Arduino. You would need to control the auger motor and water flow. Have a start button, a stop/panic button, user feedback in the form of LED's or LCD and of course the input from the scales. All seem doable for under $1000