I want an mCU that:

-can drive a piezo speaker (most will)

-has enough gpio to detect at least twelve keys (that's twelve gpios, maybe 8 if multiplexing)

-is easy to find for cheap

-run on internal oscillator without any external component.

Atmega8 is a good candidate, can be bought for $0.90on aliexpress  if you buy 10. But buying 10 is cheating. It can also be unsoldered from a $2.35 avr usb programmer, or an arduino pro min can be bought for $2.72

I found an Atmega8A on ebay for 1.04€. Ended auction don't seem to get lower than that.