The Eotech 517 or Aimpoint PRO are both in the $400 range, and are rugged, capable sighting systems. I have an Eotech EXS2, which is smaller and uses a Surefire 123 battery. I also have the Aimpoint PRO, and would generally recommend the latter. The PRO can be left on for three years continuously (5000+ hours), whereas the Eotech has about 1/10th the battery life, and goes off automatically after 4 hours. Mine are on rifles, but a shotgun would be no challenge, with an XS rail or something comparable.
For about $225, there is the Burris Fastfire III, which is somewhat harder to zero and has fewer features, but small and effective. It is small enough to mount on a pistol. It has an on/off switch, or can be turned off automatically with an opaque plastic hood, supplied with the unit. I have the Fastfire II, but the III has a battery changed from the top and click stops on the windage and elevation screws. The 4 MOA dot is clearly visible for quick target acquisition in all light levels. It is powered by a CR2032 button battery.
The Eotech and Aimpoint are submersible, so a rainy day on the hunt would not be a challenge. The Burris is weather resistant, but I wouldn't dunk it. I wouldn't trust anything cheaper to withstand the recoil or bad weather.
