I got a John Pearse transducer pickup for sale if you want it. PM me for details.
You stick it on the top of the guitar with some tape - it's the cheapest kind you can get - sounds ok but a bit trebly (but you can't expect much considering how little it cost...)
the absolute cheapest i can think of is a piezo pickup. try a searh on it. a bit of diy is needed. juz go to sim lim tower get a piezo, (they call it buzzer there) get some wires at a diy shop. a 1/4 jack. wire/solder them all together and u got a cheap piezo.
soundwise its a bit much on the mids and lows. play ard with the place ment of the piezo. u may have to use the amps EQ to suit yer taste.