The old saying "you get what you pay for" is pretty much spot on when it comes to red dots on shotguns, but I digress.
I mounted a Tru Glo Open Shot($50) on a friends gun and it sighted in very easily and the view, dot, etc were very similar to more expensive sights. One thing I don't like about many of the cheap reflex and tube style red dots is they are much higher than the more expensive reflex sights. That extra height usually doesn't allow proper cheek weld.