What is the value of a 1919 US nickel?

Its value will depend on its condition, and if it has a "D" or "S" mintmark on the back below FIVE CENTS.

In well-worn condition, but with complete date and full rims : no mintmark = $1.00 / D-mint = $8.00 / S-mint = $5.00

In medium worn condition, with about half the buffalo's horn visible : no mintmark = $2.00 / D-mint = $40.00 / S-mint = $35.00

In lightly worn condition, with clear & fully defined horn : no mintmark = $10.00 / D-mint = $170 / S-mint = $160.00

Nice, lustrous uncirculated : no mintmark = $40.00 / D-mint = $400.00 / S-mint = $375.00

The 1919 Buffalo nickel is a common date, retail values for circulated coins in average condition are $3.00-$8.00