about $0.30
Look at the date and coin again, the last year a Buffalo nickel was struck was 1938.
In circulated condition, still only 5 cents. Save for a couple key dates, even early Jefferson nickels are only worth face value.
Value is all dependent on condition and mint. The mink mark is on the back at the bottom to the left of the fasces. 1940 (all mint marks) can range from 1.50(lightly circulated) to 30(uncirculated)
According to the Standard Catalog of World Coins, the US Mint produced 11,426 proof sets in 1940 and sold them for $1.89 each. According to the same source, a 1940 proof set is currently valued at US$1,475.00. If you have one, however, I would suggest going to a reputable coin dealer for a better valuation based on the condition of your particular set.