Refurbished office PC's are pretty cheap around $150.
However Level1Techs did a good series on 'Curing Consolitis' and building really cheap PC's.
The key is to buy a recycled PC from a refurbishing shop before they refurbish it.
You can get a decent PC for $50 and have money left over for a graphics card.
Once the refurb shop touches it, the price goes up $100.
https://www.youtube.com/watch?v=Iz52pi1Y4_o