Welcome to the Forum. Lots of great info here on the various pages and in the heads of your fellow members.
If you have nothing now (no radio, no speakers, no speaker housings, etc.) the HD full-on rig is gonna be steap. About $1k for the radio kit plus dealer costs to install and setup. Here is the babble on it:
"Upgrade your Electra Glide Standard model to the latest in mobile communications technology. The Advanced Audio Radio is the first step to a total entertainment experience. Compatible with other Advanced Audio accessories, the radio features a menu-driven display that provides easy access to the many standard and available features. The radio features 6 AM and 10 FM radio presets, integrated weather band with weather alert warnings, and a single disc CD player that is compatible with CD/CDR/CDRW and MP3 format discs. MP3 compression technology allows each CD to hold up to 10 hours of music. The two fairing-mount speakers (included) are powered by a 20-watt per channel internal amplifier, and the Automatic Volume Control adjusts the volume to compensate for motorcycle speed. The complete Advanced Audio Radio Kit includes everything your dealer needs to install the AM/FM/WB/CD unit in the fairing of a 2006 Electra Glide Standard model. Dealer installation is required, labor cost not included.
Fits '06-later Electra Glide® Standard models. '06-'08 models require Frame Mount Antenna P/N 76325-06, '09 models require Antenna Frame Mount Kit P/N 76556-09 and separate purchase of antenna. '06-later models with Tour-Pak® require Tour-Pak® Antenna Kit P/N 76251-98A."
So you can see you also need some additional stuff not in the kit.
Aftermarket will run the gammit of pricing. You probably want something that will be controllable by your bar controls (assuming you have those?). My bike is an UC, so it has the OEM stuff (although I pulled the radio and set that to Radio Sound for a workover a while ago), so I do not have any first hand knowledge on the aftermarket except what I have read. Someone should be along here with some experience.
TQ