How loud is your stage volume? That will be the driver of how loud the monitors need to be. You need to be able to hear them over the loud amps and drums.
I am a keep it simple type guy. A couple of small powered 10-inch JBL speakers are what I use if I use anything. They are light and easy to carry. Plenty loud enough to be heard on stage. We usually just use them so everyone hears the vocals. For clubs, we do not mic our equipment, we just our vocals and maybe the kick drum. In my experience, you don't need a lot from monitors. For bigger festivals, they tend to mic everything and have their own monitors and run everything through them. Whenever we do festivals, I always tell them to just run vocals and kick for the monitor near me.
Like others have said, the monitors are for the band and not really for the audience. I don't think they need to be that high of quality.