What is the value of a buckwheat penny?

There's no such coin. You're probably referring to a wheat-ear cent which shows two ears of standard wheat on the reverse side. These coins' values depend very much on their date and mint mark; prices can range from a few cents for common dates after 1940 or so up to many thousands of dollars for rarer early dates.

Please look for existing questions in the form "What is the value of a [date] US cent?" to get a specific price for your coin.