I use a combination of 3 electret microphones hooked up to a home made passive mixer for use on my melodeon. The output from this set-up is then connected to a Trantec wireless mic which allows me to leap around the stage / visit the loo / go for a dance whilst i play. I'm not known as the gentlest of melodeon players, and the set up hasn't failed me in the last 10 years. Sound quality is very good.

Electret microphone inserts are quite cheap, and if you have a very basic knowledge of electronics and you can solder, a pickup system can be made for just a few pounds (or a few dollars more.........hmmm, I could use that phrase for a film title :-))

John