Here's what I've done to amplify banjos; both for me, and repair shop customers.
You go to Radio Shack or whatever electronics store you use, and you pick up plastic-cased piezo element, or buzzer as some people call them. They come in different tones and operating currants.
So far, I've used the 3000hz and 2800hz models. They cost about five bucks.
What you do is CAREFULLY break the back off the plastic casing and loosen the piezo element inside and remove it. Don't pull the leads off the crystal. If they come off, they can be carefully soldered back on with low-heat solder.
Whatcha got there is a piezo crystal that can be trimmed with scissors to whatever size you want and fit and glued onto the back of the banjo bridge. Double-sided foam tape is a good mounting medium, and cuts down on the really high frequency noise. Solder a micro-sized jack onto the leads and secure it to a banjo hook.
It's a good idea to use a pre-amp with this set-up, since the signal can be kind of crunchy. Experiment with lower frequency crystals for different tone colours.
I've used this setup on banjos, mandolins, and fiddles. They're so cheap that you can make up several of them of differing buzzer tones and see which one has the sound you like. If you break one of them, it's easy to make another one.
The neat idea here is that they can either buzz or be buzzed. It just depends whether you put currant through them, or put them on a vibrating surface to create currant. Cool, huh?
Anyway, there you go. Cheap, noisy, fun. Heck, give them as gifts.