You might want to check in the 'Amps, mics and pickups' category. There's an interesting post I think it's called 'micing with a bass amp' where a few people have had great success with clip on tie microphones (me included) The whole thing cost $13. Admittedly I have only tested it with recording and got a great sound. The big test for it is tonight at a rehearsal through a pa. Otherwise for under $100 I don't think there is really a great range if dedicated db pickups to choose from. A k&k might be around that price, they are really simple to use and install.
"....all those low frequencies vibrating through you, its gotta be good for you right?" - Captain Mission