A very good accoustic amplifier is the German made A.E.R. They have several models but, from what you say, their "Compact 60" sounds ideal. I have tried one of these and it is the best sounding amp I've come across. It isn't loaded with features/effects, very portable and has a separate input for vocals.
Most amplifiers have a "line out" to connect into the house P.A. for bigger venues and you can use your amp as a monitor.
With regard to cost...In the U.K. they retail around £550.
Another good accoustic amp is the Polytone. Again, very portable, but possibly more expensive.