Just for completeness, and in case others with very limited budgets read this, there is also the option of the DSO150 "Shell" scope from JYETech. It's a very low frequency (they claim 200KHz) single-channel scope available in kit form or assembled from Banggood (known to have genuine stock, not clones). The assembled scope with a probe currently sells for US$21 plus about $3 shipping:
In addition you would need a 9V power supply with the right barrel connector. However, I converted mine to LIPO battery power, which has the additional advantage that the scope isn't referenced to ground, so you're less likely to blow it up, and you can do things like measuring, and triggering on, the voltage drop across a mosfet.
Of course this is limited in what it can do, but I have found mine to be surprisingly useful for Arduino stuff. It works with PWM, steppers, servos, IR remotes, audio, and other similar things. I recently repaired my Disney coffee mug warmer after using the scope to detect a very brief dip in the 5V power supply which was resetting the MCU. Turned out to be a bad electrolytic capacitor (surprise!). And using its 1K test squarewave, I was able to use the scope to see the very high ESR of the cap even though the capacitance test was fairly normal.
Anyway, for under $30 including a power supply, you get a basic slow scope. And if you blow it up, you haven't lost much. It has Auto, Normal and Single triggering, but because of limited capture memory, the trigger only works at 20ms and faster per division, but that's usually not a problem. And it has data readout - frequency, etc.
If you do get one, get it from Banggood. The clones on Ebay and Aliexpress will not take firmware updates, and yes, these dinky little things do get firmware updates from time to time. And JYETech has a forum if you have questions. There's a long thread there on battery conversion options. So this is not junk. It's a quality product, with support, for a very low price. And even if you do eventually upgrade to a "real" scope, you may find yourself still using this one more often than you would expect.