Mouser has the Zero for 15$, 16$ if you want presoldered GPIO headers. 4$ for a Pico. RS Online has them for 4.25$. There are probably hundreds of vendors that offer them for the same price if you plop the term into your favorite search engine. Amazon is the last place I'd look, but then again I absolutely do not understand this mindset that online shopping == amazon that some people seem to have.
e: formatting