A team of physicists and engineers at the University of Strathclyde, in Scotland, working with a colleague from the University of Glasgow, has developed the means for 3D printing a microscope, including the lens, for under $60. In their paper posted on the bioRxiv preprint server, the group describes how they came up with the plans for the microscope and notes that the end result has a resolution strong enough to make out individual blood cells in test samples.
This was done back in 2014 with cell phones, glass beads and 3d printers and the cost was under a dollar:
https://ceramics.org/ceramic-tech-today/just-bead-it-glass-spheres-form-powerful-and-inexpensive-cell-phone-microscope-you-can-3d-print-at-home/