So starting with nothing at all, what is the total cost of ownership for the flea -cope? My guess, its over $300, (if $300 is your annual salary, your existing phone will probably not cut it). And for comparison, standalone toy oscilloscopes (<10Mhz bandwidth) go for $30 and under (some $11), with screen, on AliExpress. And a "decent" one, like OWON, is around $150.

The problem is, with a toy scope, you aren't really gonna know if what you're measuring is real. This might be useful as a kit to build to learn how to program microcontrollers, or measure audio signals (the $11 one, or the mic on your phone can do this), but a bad scope will generally cause more problems for the hobbyist. When you get near the limit of a scope's bandwidth, the signals get really messy and full of artifacts. Arduinos run at 8-16Mhz, so you're gonna hit a wall really quickly and once you can't rely on the output, the investment will be lost.