Steve Chamberlin's "Big Mess of Wires" processor took "18 months, about $1,000 and 1,253 pieces of wire" to build, measures roughly one square foot and, amazingly, computes at about the level of a Commodore 64. [Gadget Lab]
A Ph.D. student in my old university group (where I was a researcher) built a Z80-like CPU out of 74-series logic in his spare time. He got it to run at several hundred kHz. This was over 10 years ago.
The only difference from this guy's effort is he soldered it, instead of using wire-wrap and didn't get as far as adding peripheral support (it was just a CPU with a UART).