Using RS232 to light bulb or move motor
If you have a working arduino solution, you can probably reproduce it inexpensively by using your arduino to program another atmega 44/88/168/328 and placing that on a board.  See http://arduino.cc/en/Tutorial/ArduinoToBreadboard

If timing is not critical you can use the internal oscillator - meaning your system comes down to a $3-4 atmega and a serial level shifter IC.  Or potentially even a $2 attiny and soft serial.

If you only need to receive serial you can use the 1489 line receiver (its companion transmitter the 1488 is what required the extra power supplies) instead of a modern charge-pump level shifter.  At a low baud rate you might even be able to use a resistive divider into a TTL inverter, or do soft serial and modify it to understand the "upside down" waveforms that would result from not bothering with the inverter.