Weather balloon - $70 for one that can lift 600 grams. Probably overkill, but overkill is better than underkill
GoPro or similar - $200 for a secondhand one. You may already have one lying around, and it is almost certain that you have a friend with one that you can borrow
Small parachute - $40 seems to be the going rate. You could easily make your own out of scrap fabric though.
GPS tracker - $40
It costs somewhere in the range of $110 to $340 to get your own pictures of the curvature of the earth.
Or you could go the coast and look at distant ships and notice that the bottom vanishes before the top.