In order to learn more about getting the cheapest PC that would run the Witcher 3 check out this post: https://steemit.com/technology/@hwtrendsetter/the-cheapest-way-to-play-the-witcher-3
Examples of prices if you buy locally (in this case prices were found for Warsaw, Poland)
Total for the PC - 124 PLN (~29€)
Wicher 3 - 30 PLN
Game Of The Year Edition (includes all expansion packs and DLCs), new, in box
https://www.olx.pl/oferta/gra-plyta-wiedzmin-3-dziki-gon-edycja-gry-roku-nowa-CID99-IDogWfY.html
Old keyboard - 0 PLN
https://www.olx.pl/oferta/klawiatura-ps-2-CID99-IDxe22q.html
Old mouse - 2 PLN
http://allegro.pl/ShowItem2.php?item=7721246245
Monitor (not panoramic but flat-screen and includes cable) - 10 PLN
https://www.olx.pl/oferta/20monitor-gateway-CID99-IDu2S09.html
Any Windows edition offers free trial for 90 days (more than enough to complete the game).
Total for the desktop PC, monitor, peripherals and Witcher 3 game - 165 PLN (~38.50€)
Just for reference if you were a console gamer and had to buy Witcher 3 Game Of The Year Edition from PlayStation Store, that would cost you 199 PLN (~46.50€)
https://store.playstation.com/pl-pl/product/EP4497-CUSA05571_00-00000000000GOTY1