Answer:
It depends on;
a) Which African country.
b) What denomination (ie, 50 Zambia Kwacha)
c) How old.
d) The condition of the note.
To get an accurate answer, you need to ask "what is the value of a {year} {Country} {Value} note in Good/Poor/Mint condition". If this was an American note, you would ask "what is the value of a 1979 US $10 bill in mint condition".