Updated in December, 2017
My original poitn remains: if you are really looking to squeeze under $100 mark, your best bet is to look for sales (every once in a while I see Sightron S1 go on sale with a couple of models squeezing under $100), or to frequently check SWFA’s sample list, for lightly used or demo items.
Aside from that, if you shop int his price range, go with a brand that will support you when something goes wrong. I suggest Hawke (their Vantage scopes are under $100) and HiLux with their Hi-Lux Optics Professional 1.5-6×42, which is a surprisingly decent scope for a hundred bucks.
Updated in March, 2016
As of lately, I have not really stumbled onto anything that is under $100 and is actually decent.
If you are really looking to squeeze under $100 mark, your best bet is to look for sales (every once in a while I see Sightron S1 go on sale with a couple of models squeezing under $100), or to frequently check SWFA’s sample list, for lightly used or demo items.
If you are looking for a riflescope in the <$100 range, that means that you are REALLY on a budget with this project. In the world of riflescopes, there are a lot of cheap scopes and with most of them you get what you pay for. In this price range, a riflescope should be expected to do one thing and one thing only: hold zero. Everything else is gravy.
If you are on a budget, go for a simple scope of moderate magnification. In this price range, you do not want the scope to have a lot of fancy features. That just means that the manufacturer paid less attention to the basics.
Here are the scopes that I do not necessarily like but that seem to hold up OK and cost $100 or less:
This is the February 2011 update since the scopes I selected for this last year are either no longer available or have moved up in price a touch.
For a rimfire, $50 Swift Reliant 4×32 is a surprisingly decent choice as is the 4×32 version on Nikon ProStaff.
Now, this does not mean that all other cheap scopes are bad. However, most of them are, and the scopes I listed above are the ones I have enough mileage with to be able to recommend.