A quick and nasty way is to use a resistor to limit the current,
two diodes (1n4148) between the 0v and 5v to form a clamp and an inverter. The inverter would want to be a schmitt variety (74hc14 etc) to give some hysteresis. Sure it works and I've seen a few other variants in hobby magazines and on the web, but does it comply with the required standards? Most likely not, so that's why there's specialised chips to do the job. You can also use the output of an inverter as your TxD signal - it works in MOST cases!
If cost is an issue, remember there's always the old 1488/1489 chips along with CMOS variants. Nowadays, most people use the MAX232 style devices as you only need a 5v supply.