How cheap do you need? You could find an older Boomerang for under $200 or a Boss RC20 XL for $150 or less, or the RC2 or RC3 for $100? Digitechs are good too and can be found in the same price range but I'd have to vote for the Boomerang.
Are there kits available? Unless you are already very comfortable designing low-noise mixed-signal circuitry AND you have the necessary programming skills to write the code for a fast micro or DSP you might find building one from scratch to be a bit of a challenge.