There was an elderly gentleman that was hard of hearing that used to come to one of our jams. He wanted to be able to hear what he was playing so he didn't throw everyone off. I put together a system for him using a $14.99 Radio Shack personal amplifier. You will need some sort of pickup (I think I used a suction cup phone recording mic) and an earpiece (I used one from an old transistor radio). I wasn't HiFi but it worked.
Here is a link to the amp.
"bargain instruments are no bargains if you can't play them"