cheapest job would be an extension cable from the plug on the back of your PC. basically, it has a headphone jack on one end, and a plug hole on the other end. where you'd buy that in town i'd be dammed, but i have one like that.
alternatively, buy a mixer or amp/preamp with a headphone jack in it.
"Our Model MK-5 Computer Audio Switch Box is a great solution for conveniently connecting and using your computer's audio input and output capabilities. It simply ends the hassle and inconvenience associated with using both speakers and headphones on your computer audio system. With the MK-5 Switch Box, switching between speakers and headphones is now very EASY, FAST, and EFFORTLESS!
Plus, as an added feature, the MK-5 provides an easy access "front panel style" solution for connecting a microphone to your computer. Installation is simple. You'll no longer need to go through and decipher that rats nest of cables and plugs in the back of your computer just to hook up a microphone, a pair of headphones, or to reconnect your speakers. The MK-5 makes headphone, speaker, and microphone connection & selection simple, fast, and VERY CONVENIENT."
From the Manufacturer
"With the Plantronics PC Headset Speaker Switch you can now can use your computer speakers and headset at the same time. No more fumbling around with plugs and wires when you want to use your headset instead of your speakers. The Plantronics PC Headset Speaker switch allows you to easily swap between the two at the touch of a button. Its small size takes up very little space. This switch is a convenient solution to your headset needs that takes only seconds to install. Just plug it into the sound card and you're ready to go. Compatible with any PC or Macintosh computer that has a sound card."
'and the rest is hysteria......'