Quantity   Component name
1 × Arduino Pro Mini ~$3 each on <a target="_blank" rel="noopener noreferrer" href="http://aliexpress.com">aliexpress.com</a>
1 × nRF24l01 board ~$1 each on <a target="_blank" rel="noopener noreferrer" href="http://aliexpress.com">aliexpress.com</a>
1 × Custom board $9 for 6 at OSHPark
2 × 1 uF Cap 0603 couple pennies
1 × 100 uF cap 0603 couple pennies
1 × 662k Transistor 5.0v to 3.3v couple pennies
1 × Micro-USB female connector a dime
1 × Long PTH header strips $0.78 for a strip of 40 (and that was from SparkFun so they are probably cheaper elsewhere)