Updated in April, 2020
If you are shopping for a riflescope under $100, you are on a sever budget crunch. $100 is a lot of money for a lot of things, but for riflescopes that is less than pocket change.
If you are really looking to squeeze under $100 mark, your best bet is to look for sales (sometimes Sig or SIghtron will drop some discontinued product into the $100 range), or to frequently check SWFA’s sample list, for lightly used or demo items.
Aside from that, if you shop in this price range, go with a brand that will support you when something goes wrong. I suggest Hawke Vantage, Athlon Neos and HiLux which makes a couple is a surprisingly decent scopes for a hundred bucks. Swift Reliant 4×32 is a decent simple 4x scope.
If you are looking for a magnifier, Primary Arms has one for around $100. It is not great, but serviceable.
There is a ton of various Barskas, BSAs, Truglos, etc in this price range. They are all crap and I would not touch one with a ten foot pole.
Hawke, Athlon and HiLux will not leave you high and dry if there are issues.