Wikipedia's entry on relative values of the chess pieces
Here's the wiki on relative values of the chess pieces. Many different ones are suggested, as can be seen from the large table given at one point. It may be surprising for experienced players just how often a single bishop is rated as equal to a single knight on average; computer studies have led at least some authorities to believe that this is in fact true:
https://en.wikipedia.org/wiki/Chess_...relative_value
There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.
Hamlet (1.5.167-8), Hamlet to Horatio.