We present a concept study for a novel All Sky Monitor experiment employing very limited resources. Our experience in designing, building and testing SuperAGILE - the hard X-ray imager for the AGILE mission - has demonstrated the possibility to develop a medium-sensitivity, wide field imager, with (at launch stage) ~5.5 kg weight, 12 Watts power and 0.04 cubic meters volume. With these few resources, it can provide crossed one-dimensional images of 1/10th of the sky, with on-axis 6 arcminutes angular resolution and ~10 mCrab 1-day sensitivity in the 15-45 keV energy range. In this paper we introduce to the ASPEX (All Sky Project for Extraterrestrial X-rays) project and show how a much more efficient All Sky Monitor can now be designed using the same approach and techniques, overcoming a number of severe limitations suffered by SuperAGILE due to the context of the AGILE mission, for which it was designed. The low resources and its efficiency in localizing X-ray transients and in long-term monitoring the steady X-ray sky, make ASPEX a suitable option for several new mission concepts (e.g., PHAROS, ESTREMO, ...).