About $1400.00 to purchase the upgrade for rear speakers, wire loom, and center consol change to put HD plug to hook in helmet speakers....
Or about $60.00 to buy a part from J&M. I think it is something-38. The J&M rep will know if you call. It was original for old Goldwings. It splices into the speaker wires in the fairing and you have to drill a whole to put the plug in the fairing (sort of like the HD lighter). You plug the helmet speakers inn and any J&M headset. Only draw back is the fairing speakers still work. But you control the helmet speakers with the radio volume control.
If you want a minute jack for earbud speakers, you could probably have someone make the same kind of splice kit with a minite jack.
Last, J&M also makes a kit for putting in a "y" to add the same for a side car. I bought that also. I use it when I have my wife on the back for her headset...when she is not on her Sportster.