Less than $2, just get one some of these...

https://www.aliexpress.com/item/Free-shipping-1pcs-Digispark...

Wack a buzzer on them, add some code and then leave them in a usb port near your victim. Easy.

If you get the bits directly from China and put them together yourself, you could probably make them for a few dollars a piece in quantity.

See my top level comment - about $10 per unit.