Not quite $100 but I see the aging yet highly-regarded, video-centric Panasonic GH4 sold used for as little as $200 without a lens. This is the camera many small-midsize video production outfits have stuck to for a >decade. There are many (often fairly good) generic lenses for the 4/3” sensor mount it uses, and the camera is known for having a clean Full HD HDMI out. I can see building a setup like this for as little as $300-400. Add a $40-$100 LED light mounted on the camera and you’ve improved your video presence by 10x for less than $500.

*Edited for grammar.