IMO the best 2 budget optics are the Bushnell TRS-25 and the Primary Arms Micro Dot.
I have no personal experience with the PA red dot, I am just going off the large amount of positive feedback I have seen with zero negative feedback.
The Bushnell TRS-25 I do have first hand experience with because I have one on my SGL21. For $80 I think you would be extremely hard pressed to find a more durable red dot, they also have good clear glass, and the red dot is pretty crisp.
I think you will find these 2 red dots to be the most recommended budget optics.
If you have the cash to blow then I would recommend an Aimpoint Micro, a product as equally durable as the Kalashnikov.