You mean the cheapest period? Probably something like this ... note that this is maintaining some semblance of balance in the system; e.g., not dropping down to an i3 which could save you another $60-$70 at a great sacrifice in performance. And also keeping out anything that's so poor quality that it would be dumb, like the bottom-of-the-barrel PSUs.
Would I do a couple things differently if it was my own machine, like a better PSU and slightly more up-to-date motherboard? Hell yes. But if cheap is all you want, look here.