My setupOnce upon a time our band needed a device that can play pieces of music like background fills, breaks etcetera, "because a musician only has one pair of hands".
The musician(s) should be able to play & record the extra parts in advance and have another musician start the playback on the proper time (in our case this would usually be via the drum pads midi out).

So the starting requirement was a box that plays a sound file of any length completely and only once, to be triggered via midi. This could be achieved with some adaptations of the sampler box by Joseph Ernest.
As it happens, this was just the start ...

So what's it now in non-commercial terms:

On the right you find an indication of what I have used for the initial trial, adding up to about € 80,-.

This proved you can make a box with functions only available in expensive and complex equipment, but much cheaper than the cheapest sampler boxes on the market.
BUT: very inexpensive stuff is totally unguaranteed. Also some functions (like freeverb) require PI3.

Currently I use a PI3 and PI3B+ with more solid midi and audio interfaces, which doubled the price.
PI4 is not supported, on the wishlist.
Raspberry PI 2 37 €
Case 8 €
Power adapter 3 €
Jumper wires 6 € (this was a pack)
USB Midi cable 5 €
USB Sound adapter  3,5 €
2x16 display 4 €
Buttons 3 €
Trimmers 1 €
Shipping costs 7 €

Credits to Credits:
The project is based on the sampler box by Joseph Ernest, extended with
the 16x2 LCD interface by Rahul Kar, accurate velocity control by Erik, voices and several nice tricks by Alex MacRae, randomizing samples and I2C-LCD by David Hilowitz, freeverb by Jezar at Dreampoint, D'Angelo Moog ladderfilter code via Dimitri Diakopoulos, AutoWah by Daniel Zenco, delayline coding by Gabriel Rivas, Simple Compressor class by Citizen Chunk, Midi file toolkit by vishnubob, auto-hotspot logic by Roboberry, several extras by GitHub contributors, as well as various ideas and snippets I just bumped into.
Thank you guys.
Also thanks to Reza Yazdi for guiding me into the world of Persian music.
I've glued these things/ideas together and added lots of cool things in the process.

If you have extra ideas, please share them on or via GitHub pull.

Audio Interfaces: USB Audio interface

MIDI Interfaces: USB midi interface

MIDI controllers: Midi-controllers