For optics, what are you planning using the rifle for. CQB or distance shooting. For anything under 100 yards my personal preference would be an Aimpoint, but there are several other red dot or holographic sights that are just as good. If you're looking for magnification there are several other options available, Vortex, Trijicon, Nikon, Bushnell, and Night Force.
For the better quality you can expect to pay $350 up to $2000. Unless you can get a great deal on a used EoTech, Aimpoint, or Trijicon, I wouldn't buy an optic below $300. The way to look at it is, are you willing to bet your life that it might work?
Just to give you an example on glass prices, my scope was $200 more than my .308 rifle cost.