Focus less on the cost of the printer and more on the cost of materials. If your second grader is anything like the 2nd graders I have known, there will be more than one incident of "I googled and found 3d models of all the Pokemon/MLP/Pixar characters and I've been printing them for 6 hours and we need more printing filament". You want really cheap supplies, not the stuff your printing car parts or ghost guns or whatever out of.
There are tons of really cheap 'USB Microscopes' on Alibaba/Amazon that are probably more fun/accessible than a traditional glass microscope. I know I've gotten my young nieces/nephews ones that they can carry around and point at things, ones with goosenecks that they can stick down holes, ones that work underwater, etc., all for a ~US$20 or less. Most have BT so they can throw the images to an old iPad or retired Android tablet. They loved them.