Guess if you can afford a $2000 scanner you can afford a $200 used iPhone.

Not really. The thing with Apple devices is that you can be relatively sure it will work and you only have to test against a couple devices. For Android handsets? Different and vastly larger beast.

There's so much that may or may not work: the USB port must support OTG/Host on the hardware side, which is already a hit-and-miss. The port must be able to supply the power required for the ultrasound. Then the Android stack must support USB OTG - and that must have been properly tested during manufacturing. And then the vendor must have compiled the Linux kernel below Android with support for uvcvideo (which is not standard enabled in cheap phones). Then the CPU and video unit must be powerful enough to run the software with video playback.

android is currently under development

I too doubt iPhone is the cheapest capable smartphone and the most cost-effective way to do whatever it can do.