I can understand how molecule-coated gold nanoparticles suspended or sinking in a blood sample can be detected by light at a cost of only $2 per test, but I would think that the process of coating the nanoparticles with the appropriate antibodies, antigens, etc. would be quite a bit more costly. Perhaps those ingredients are prepared in volume keeping them cheap?

COVID lateral flow tests use gold nanoparticles to make the purple lines.

I assume this is the same, but for a wider array of diseases.