A $100 isn’t much money for a computer network project especially since networking means more than one computer. When I was starting out I did setup a practice network, actually I did this several times out of any hardware that I could beg, borrow or steal (not really steal). Later in my career when I was giving interviews to new technicians I found out how common it was for techs to build practice networks at home. Anyway back to your project nowadays thanks to virtualization you could build a network on one piece of good hardware making your $100 budget for this project difficult but not to impossible.
I ‘d say buy the best used computer you can find and scrounge up at least 32gb of ram. Then install VMware ESXI host software (free) use youtube videos and free tutorials to do this. Then download Windows Server 6m free evaluation software. If your $100 piece of crap hardware can support it install two instances of windows server make 1 a DC and the other a member server. Then one client computer Windows 7 or 10 free eval copies and a free Linux software for a web server now you have the basics of a working network to practice on.