Not necessarily the cheapest possible options, but cheap enough and fairly quick to get going:
If you want a MIDI pad controller, the Korg nanoPAD 2 [1] is $65 new (and often pops up used) and powered entirely off USB. You can then set up Sitala [2] to listen for notes from the pad and drag-and-drop samples onto each note.
If you want keyboard-style keys, the Pimoroni Keybow kit [2] is $57 - there is a bit of assembly, but no soldering.
I've seen nanopads (1 and 2) go for $20 fairly often; the kind of thing that's so cheap I'm always tempted to get it with minimal actual use cases for it.