+1 to aluminum w/framework.
down n' dirty cheapest would be painted MDO with graphics.
we do large-scale architectural signage, so I'm more inclined to say build from scratch and do something really great while you're making the effort.
also, whatever you do, make sure you get a PERMIT.
some towns/communities even require them for banners these days. "graphic changes" to an existing
sign? etc. etc. you bet, permit.
additional note: cheap is crap. Make a couple cheap banners, bake some pies, and throw a fundraiser at the church.