That's too generic of a question to answer, you need to specify if you have any parts, monitor res/hz & even settings. "Cheapest" would probably be a older prebuilt that you add a decent GPU into it.
It is possible to get used workstation class PC's for under $100.
Find a shop that recycles office equipment and buy one before they refurbish it.
Once they touch it the price more than doubles.
The monitor and some stuff iss fine , just need a chepeasg build in the cpu itself like (ram,mobo,graphics card and cpu) and the game i want to play was pubg