What is the value of an 1838 buffalo nickel?

I assume you mean 1938 because buffalo nickels weren't issued in 1838. In worn condition, it's worth about $3 retail; $4 or $5 if almost uncirculated. 1938 was the last year of issue for that design. They were only struck at the Denver mint because Philadelphia and San Francisco were striking the new Jefferson design. However, some dies were prepared for San Francisco buffalo nickels but were never used. The "S" mint mark was overpunched with a "D" creating a second variety. If you can clearly see the D and S mixed together in the mint mark, your coin could be worth 2 or 3 times the values I gave above.