Here's what I did for my Shadow 750. I wanted to be able to talk to my friends on their Goldwings, but I didn't want to spend a fortune. I realize you have a CB on hand, but maybe this will spark some ideas. You might look at using the external speaker jack on the CB and run that to the headphones... just don't crank it up too terribly high.
Anyway, this was my set-up until I decided to buy a 2000 Goldwing SE... which solved the CB, Intercom, and uncomfortable seat problem on the 750.
I bought a 40-channel handheld walkie-talkie... Midland 75-822. Look around and you can find a decent price. I think it was about $70. It comes with all 40 channels, a rubber duckie antenna, battery adpaters, and a 12-volt adapter. It has the weather band, memory, and a power-saving mode. It has a 1/8" headphone out jack and 3/32" mic-in jack that responds to a PTT key. Plus, it's handheld so you can take it with you.
For the headphones I bought a cheap set of Sony headphones that wrap around the back of your head for about $15 at Best Buy. I took the pliers and knife to them and removed the headphone elements from the mounting -- you don't have to monkey with the wiring... just cut the plastic parts away from the part you wrap around your head. Be careful or you might nick the wires. When you're done you'll have two speaker elements and a 1/8" plug all ready to go. They won't be super loud, but they'll work -- we're not talking high fidelity audio, but it's listenable. Mount those in your helmet -- get them as close to your ears as you can.
The mic is the fun part. Run down to your local trucker supplier shop, like a Flying J or a Pilot, and pick up a cheap 4-pin handheld mic like you see on a CB. You need to make an adapter to use this with the walkie-talkie, but it's easy. The goal is to get pins 1 and 2 on the handheld mic to the tip and sleeve of a 3/32" plug. You can find a 3/32" mono plug and a 4-pin male micreceptacle at radio shack. Doesn't matter if pin 1 or 2 go to the tip or the ring.
I soldered pin 1 of the mic receptable to the tip of the 3/32" plug and pin 2 of the mic receptable to the sleeve of the 3/32" plug (or vice versa.. whatever you like) with a length of wire, then mounted the 4-pinreceptacle in a project box with the 3/32" plug dangling out the other side.
Plug the headphones into the CB, plug the 3/32" adapter into the CB, plug the handheld mic into the adapter. Turn on the CB and adjust the volume. To talk, grab the handheld, press the key, and talk. It doesn't pick up wind -- worked fine for me behind a small windshield and face full of wind.
Disadvantanges: You have to free up a hand to talk.
Advantages: The CB is the most expensive part. But for the price you get a 12V cigarette lighteradapter with a whip antenna adapter, an adapter for plain AA batteries, and an adapter that takes a scad of rechargable AA's and the charger. You can go all day on one charge.
Another advantage: handheld mics look cool. Well, atleast I think they do on a motorcycle. Several times a tailgater wouldn't back off, so when I called ahead to suggest we let the moron pass, the car backed off. I guess they thought a handheld meant business or something. Looked like CHiPs maybe.
I'll leave it up to you how you hang the the mic. I suggest you mount it on your right and key with your left. It's easier to reach to your right with your left hand if mounting on the handlebars. As for mounting the radio and the adapter... I can only suggest being creative. On my 750 I used U-bolts and a 90 degree inside cornerbrace, mounted the radio by the belt clip on the brace and velcro'd the whole affair together.
I used this set-up ona 500 mile ride with friends and it worked fine. It takes a little getting used to and it's no good if you plan on talking in the twisties. But it's cheap. The mic was about $20, the parts were about $10, the CB was about $70. Plus, when all is said and done... you can still use the CB elsewhere.
To get another signal in there (like an iPod or radio or whatever), you could probably use a Y adapter in line with the CB and a music player, and both feed the headphones. I've not tried that, but it should work.
Is it ideal? No.
Is it cheap. Yes.
Will it work with a full-face helmet? Yes, but you'll feel silly holding a mic in front of your face with the visor up. (That lead me to a 3/4 on the road-trip.)