The cheapest CPU I'd recommend would be the Athlon X4 860K, but you would need a decent motherboard and an aftermarket CPU cooler so you can overclock it.
For Intel, I'd look at the i3 4170 or the new Skylake 6300 as a minimum. (and if you are going with the Skylake, get a Haswel i5 instead)
Reality has passed dual core Intel CPU's by, for gaming.