About the cost:
A $9 C.H.I.P. (virtually un-google-able name, btw, absolutely terrible for noobs) comes with a standard RCA video cable. It has built-in WiFi and 4GB of NAND storage.
You need peripherals. Using USB stuff means you'll need a powered USB adapter (<$10 on Amazon). You can also use bluetooth. I like the bluetooth keyboards with a built-in trackpad. That's $20 and great for ease of use and flexibility. You can pop the chip on an empty video input (because most other devices are using HDMI) and just keep the keyboard on the coffee table. No cables, no mess.
You'll need a USB power supply for the device itself. I find it hard to believe that most people can't scrounge one up from an old smartphone, or is able to find one at Goodwill, but let's call that $10.
For monitor, used is definitely the way to go. CRT displays are practically free, as well as older liquid crystals. If you really need a new device, you can definitely spend less than $50 as the resolution of the device is still going to be the limiting factor.
So, with shipping for the CHIP, that's $35 for the average person that does have a USB power supply and junky TV.