RED DOT:
If you are seeking a Red dot and want the cheapest acceptable option the Primary Arms Aimpoint micro knockoff is the way to go. I got mine used for 60.00 locally. IT is MUCH more stout than the Bushnell TRS25. I broke the mount on the TRS25 in 10 minutes of use.
If you have 300.00 to spend then you can usually find a used aimpoint PRO for 300-325 if you hunt AR15.com.
If you have a slightly higher budget then I would try the Aimpoint Micro. I don't have one. But it seems to be all the rage right now. I have heard a lot of good and zero bad. I think they are just under 600 bucks? Ryan?
MAGNIFIED:
Budget will determine a lot here. I think a good middle-road optic is the vortex VIPER PST in either 1-4 or 2.5-10 (FFP). I can always see what I can do as far as price goes. I can get a deal at vortex as long as you don't mind the wait.
If your budget allows, then you can get into some serious optics that cost some serious jack. But again, budget and intended-use are determining factors.