If your farthing looks like the one in the picture, i.e., a nice looking coin, it may draw $1 US dollar from a collector who wants to add it to his or her collection. Otherwise, it is worth less. A fully uncirculated specimen, that is, one that passed from the mint directly into a coin collection, would probably bring $3 to $5.

The farthing was Great Britain's smallest denomination, with 4 farthings in a pence (penny), 12 pence in a shilling, 20 shillings in a pound. These modern 'wren' farthings have the reigning monach on the front:

GEORGE VI, 1937 to 1952
ELIZABETH II, 1953 to 1956

They are all low value:

worn: less than $1 US dollar approximate catalog value
average circulated: less than $1
well preserved: $2
fully uncirculated: $5

The farthings dated 1938, 1939 and 1956 were struck in smaller numbers than others (though still in the millions) and as such command a slightly higher value of about $10 when in fully uncirculated condition.

For more that you'll ever want to know about farthings, go to AboutFarthings.com.