Hi sir, I'm a mechatronics engineer and I have experience with electronics, pcb and case design, all areas in relation with your project. First, for mass production you DON'T need and arduino, just a microcontroller, a LCD and little amount of analog components. Arduino is a little expensive (there is other boards like TexasInstruments LaunchPad at 9.99USD) and is a little big for your porpusses. So at this time I think final price for each one would be 30USD(Just electronics). Contact me if you have any question. Regards, Hector.