Sigghhh... I am a live sound engineer amongst my other failings. You know what I frequently see on contract riders? NO BEHRINGER EQUIPMENT. There is a reason. The stuff always is well designed, great feature pack, and sounds all right out of the box... but it is NOT reliable.
You simply can't get an acceptable microphone for the money you're talking. My suggestion is save your pennies. Sorry but that's just how it is. Besides, the SM48 is a vocal mic which is NOT the same thing as an instrument mic. Worse, it's a vocal mic that I wouldn't hand to a singer. Maybe for a speaker, but never a singer. Soundwise my favorite passive instrument mic is the Sennheiser 609, but I've had a couple of them fail on me so I don't use them. Save your money and get a Shure SM57 if you must use a passive microphone.
Honestly tho... I wouldn't put a passive on an acoustic instrument. A condenser is the way to go for this. For live pro sound I go for the Shure SM81, but that is probably too rich for your blood even if you save. Want a great deal? The AKG Peception 170 is a real sleeper for less than $100. Great bargain. It needs phantom power but if your console doesn't have that it wasn't worth buying in the first place.
@Andrew Fryer: holy hell, if you cant afford a small PA from your gigs you must not be getting much. And to be honest... nobody sounds better as an acoustic group. If you walked out front you'd probably get seriously depressed at how off-balance a lot of things are. A mixed sound coming out of PA speakers (assuming you buy good gear) is DEFINITELY going to sound better than what you're doing. And yes... your PG81 will sound MUCH BETTER with phantom.