The best acoustic guitar amp I've heard (bar none) is the SWR Srawberry Blonde.
The California Blonde is a slightly larger amp with an XLR mic input usable as a micro PA, which sounds very similar. The Strawberry Blonde, however, is a $500 amp, a little out of your price range.The requirements for an acoustic guitat amp require high-z/ line level inputs, low distortion (unlike most electric guitar amps), and broad spectrum frequency response (also unlike most guitar amps). Often, I find, the best cheap acoustic guitar amp is a small PA, which often meets the above requirements. Mostly, I use a Fender PD250 Passport portable PA. When viewed as a 250 watt PA, it may be a little weak on bass, and definitely weak on power. When viewed as a 250 watt acoustic guitar amp, it kicks ass! Also out of your price range. Check this out-
www.8thstreet.com/product.asp?ProductCode=9917&Category=Package_Systems
As a PA, this thing is a pussy. As an acoustic guitar amp, it works just fine. The other thing- if you use an acoustic plugged into a guitar amp, use a very clean amp, and consider a soundhole magnetic pickup like a DiMarzio, a Fishman Rare Earth, or a Dean Markley. Electric amps were designed for magnetic pickups, not piezos or mics. It won't sound like an acoustic guitar, but that doesn't mean it won't necessarily sound good. Leo Kottke's been using that magnetic pickup sound for years. If you can't afford a Strawberry Blonde, jack the acoustic into a line in on whatever PA you have. And whatever you do, don't buy a Crate acoustic amp. If you do, you'll remember I told you so.-Richie