Updated in April, 2020
This gives you a surprisingly larger number of options than the $100 cutoff, but my general recommendations of sticking with strong customers support and looking at used products remain.
With new products, for hunting and similar general purpose use, your best bet under $150 is probably with Sightron S1 and occasional discoutned Sig Whiskey3 and Burris Fullfield II.
HiLux 4-16×50 is fairly decent as well.
Swift Reliant 4×32 and 4×40 scopes should be at the top of your list if you like fixed power designs.
With red dots, the situation is better since you have a couple of really decent options: Vortex Crossfire and Sig Romeo5.
Good magnifiers start above $200, so the recommendation here is the same as before: around $100 you can pick up something from Primary Arms that is unexceptional, but serviceable.
There aren’t really any new LPVOs or prismatic scopes in this price range that are worth buying in my opinion, so if you are looking to set-up an AR-15 for general purpose use, your options are pretty limited. If defensive use is on the menu, one of the red sights I mentioned above should be at the top of your list. For range plinking at longer distance, you will have to drop some extra cash for a magnifier.