just as an idea, here's what i did. i built one from scratch.
i got a plastic ammo box from academy sports for 5$. put an old car CD player in it. got it from a buddy for free. came out of a mitsubishi! hooked up a 12 volt rechargable battery to it (cost me 20$ with charger). got 100 feet of wire and a loudspeaker from radio shack for around 10$. BAM... i got a varmint caller

i found a website where you can download calls for free in CD format. burned about 100 different ones to a cd and i'm good to go. all i do is string the speaker out into the field 100 feet from where i sit, and crack up the volume. it's worked like a charm for the past 2 years. i think i have 40$ invested in it. my buddy has a foxpro, and we just end up using mine when we go because now i have about 10 CD's FULL of different calls, and mine gets MUCH louder.