What is the value 1968 s proof nickel?

In uncirculated condition it might sell for anywhere between 10 cents and $4.00 for a coin of moderate to average quality (MS60 to MS64). A top-quality (MS68) 1962-D nickel could bring as much as a few hundred dollars, but these are few and far between.

A note on terminology: Liberty nickels were struck from 1883 to 1912 and carried a picture of Miss Liberty on the front. While your coin has the Word LIBERTY on it, so does nearly every other current American coin so that's not a distinguishing name or feature. The coin is standardly called a Jefferson nickel.