Holosuns are pretty nice for the money. My personal favorite is finding a deal on a used aimpoint comp m4. They can be found $400-500 depending on condition and are incredible optics.
The MRO and PRO are also great optics thag ive used and cheaper options.
Clyde armory had a solid deal on the m2. Think it was like 220 with a mount. I regret not buying one. Aimpoint makes some great stuff.