Below I will give options for purchasing, making and upcycling for your equipment. You can definitely make a great at home gym for just $100.
What you will need to be able to do well-rounded and challenging workouts.
2 moderate weights
1 heavy weight
A box, bench or stool
An Overhead Bar
An Overhead Bar and or Suspension Trainer
2 Moderate Weights:
Purchase: two dumbbells or Kettlebells that you can lift overhead multiple times. ($50- $100)
Make/ Upcycle: Laundry detergent bottles filled with water, gravel or sand depending on how heavy you want it. ($0-50)
1 Heavy Weight:
Purchase: Barbell or Heavier KB ($75 +)
Make/ Upcycle: Find or purchase an old duffle bag or backpack (I use the canvas ones found at the army surplus store). Fill with rubber much, ziplocks of sand or bladders of water. ($0 - $50)
A box, bench or stool:
Purchase: a workout bench or plyo box ($90 - $200)
Make: make your own plyo box ($40 - medium/hard)
Upcycle: find a sturdy bench, chair or stool already in your home.
An Overhead Bar
Purchase: Buy a door jam, wall or ceiling mounted pull up bar ($79-150)
Make: Buy some bulk piping and mount to some studs: ($40 - moderate) https://dreamdesigndiy.com/ceiling-mounted-pull-up-bar/
Suspension Trainer:
Purchase: I have some for sale that I purchased bulk or you can find these all over the internet.($60-149)
Make: You can make your own with as little as some webbing. https://youtu.be/IAmnLlwNloU ($10 - easy)
(If you have a pull-up bar you can mount to that. Otherwise you can attach a bracket to a stud or use a door jam.)
Optional add ons.
The longer you workout at home the more you will seek variety. Here are some other easy to access add-ons for variety and specialization.
2x4 or balance beam
Resistance bands