That's already been done. Its called a DEC PDP-8.
Between $3 and $21.6. In the future please add a condition of the coin so I can value it more precisely for you.
0 + 0 = 0 0 000 0000 1 + 1 = 2 1 001 0010 2 + 2 = 4 2 010 0100 3 + 3 = 6 3 011 0110 4 + 4 = 8 4 100 1000 5 + 5 = 10 5 101 1010 6 + 6 = 12 6 110 1100 7 + 7 = 14 7 111 1110