Humans have been building instruments for tens of thousands of years! Refined musical instruments can take tons of training and experience to build. These instruments are cheap and easy and take very little training to produce (parental support may be advised).
The Balloon Drum – this might be the most versatile and satisfying instrument you can make (for the least amount of money).
Rhubarb Flute – the rhubarb plant makes a tall stalk, which is easy to transform into this ancient instrument.
Pegboard – all sorts of instruments can be built on pegboard!
World’s Cheapest Pipe Organ – Want more toots in your life? Turn that drawer of recorders into an affordable pipe organ.