The lowest cost keyboard input device is the IR Receiver. I use them on most of my projects. Like robots, kitchen timers, weather displays, calculators, audio and video switchers, general input and a lot more. At $0.66 each, a great buy from my source:
I use my TV remote or DVR remote as it’s smaller. I also have a few from devices I no longer have.
Together with a pro mini, can’t beat the price for reliability and ease of use.
Tell us about your IR Receiver application.