It depends on your definition of a powered mixer. What the industry calls a powered mixer, is not what you want. They are very limited in mixing options but they contain power amplifiers for driving speakers. You want a standard recording or P.A. mixer without speaker outputs. A Behringer mixer can be had for as little as $40. I just got their model MXB1002 which is 10 channels with a stereo output and can be powered by two 9V batteries for field use, all for $100. They have incredible values for their low cost mixers.
Dave