I was just checking amazon and seen this one for $39.95 Plugable USB 2.0 Digital Microscope with Flexible Arm Observation Stand for Windows, Mac, Linux (2MP, 10x-250x Magnification)
For digital microscopes, the "magnification" includes the ratio of the screen size to the sensor size in addition to the optical magnification. The optical mag on that image is about 2x, while the ratio of screen size to sensor size is about 30x, so the microscope would report a magnification of perhaps 60x for the image. A 250x magnification would then be more like 8x optical, which would zoom nicely in to see a good magnified view of the mintmark.
There's a sort of dirty little secret at play here with USB microscope "magnification." You measure that ultimate "magnification" with a ruler on your monitor screen, because it's counting the magnification from showing up on your monitor in the total. My 2560x1440 screen would not offer as much "magnification" as a 1920x1080 screen (for a coin image 1080 pixels or smaller) because it'd be physically smaller on my screen than on the 1080. Unless, of course, that 1080 screen was down around 22-24" instead of my 27", at which point I would be getting more "magnification." Think about it. As "250x magnification" a Mint Mark would be 10" wide. You're not getting "250x magnification."