CARGUY2244 summed it up pretty well. You really do get what you pay for. I've had a bunch of "cheap"( and only one was a $40 sight, the rest big name and over $100) sights, and most all of them failed before to long with moderate use. If I had back the money I wasted, I'd have enough to buy a couple more Aimpoints.
Do yourself a favor, spend more than you can afford, cry once, and get something good. Aimpoint or Eotech are your best bet. I like the Aimpoints better, but many like the Eotech.
Also keep in mind, a decent mount or ring is going to cost you what a "cheap" dot will, and you have to add that into the mix.