rrwz7c - It can be done for almost no money & 30 minutes of your time (depending on what bike you have). I did something similar to what you are looking for on my '14 SGS, but I made mine Bluetooth instead of hardwired. See my post #12 in this thread
http://streetglide.com/showthread.php?2353-Speakers-in-helmet&p=41383#post41383
If your bike already has the headphone/mike jack on it, there is a 7-pin to 3.5mm adaptor already on the market. If not, and if you are not afraid of soldering 3 wires into the radio harness, all you would need is a 3.5mm male to female extension cable (a few bucks at Wally World). You would just cut the male end off of the cable, solder the wires into the radio harness at the back of the radio & run to cable to wherever you want the female jack to be. Set the radio to send the signals to the headphones in lieu of the speakers & you're good to go!
T