I use Signal Spy for iPad for audio signals. It has an oscilloscope and a spectrum analyzer. I find that a spectrum analyzer contains more useful information than an oscilloscope for musical applications, and your $100 handheld o-scopes don't do that.
The case for the spectrum analyzer is pretty easy: It's hard to "see" the correlation between waveshape on a scope and sound without a lot of experience. A square wave might not look very square on the scope, but when you flip over to the spectral analyzer, you see the expected peaks at odd-numbered harmonics. Plus you can see all the extra harmonics introduced at lower levels. IMO, it's far more valuable information, even though it's probably not as cool-looking.
I think this is especially true of DX7 synthesis, where you want to see how your levels and ratios are affecting spectrum. You can approach the shapes you want without so much hunting in the dark.