Honestly, the cheapest red dot I would buy is the Bushnell Holosight. Eventually those cheap sights will break and they are not as good as the other sights such as the Aimpoints or EOTechs. What is the max you are willing to pay?
Edit to add: I'm not the type to tell anyone to spend hundreds of dollars on equipment because everyone is different and everyone has diffeent budgets, but when it comes to Red Dot Sights, I feel that you will be throwing away your money if you buy the cheap Red Dot Sights.