For this project only, the cost is under 10$ in parts but you need to buy minimum qty of each item so you need to spend more $$$
for man/hours.... $$$$$$ it take alot of time.
equipment... $$$$$ oscilloscope, development board (for microcontroller), soldering iron, laminator(for pcb), tools, ect...
so if you start from nothing and you want to build a project like this you'll have to spend... 500$ and over ;)