I've found that the BN is about halfway between the R and Q in value, 6.5
pawns for Spielmann values. (I agree with Ralph Betza that Spielmann
values are more correct than the beginner's 1-3-5-9 scale.)
Ralph methods rate it at 4 atoms = 6.5 pawns. Note the following:
1 atom piece (Wazir) = 1.5 pawns
2 atom piece (Knight) = 3.0 pawns
3 atom piece (Rook) = 4.5 pawns
4 atom piece (BN) = 6.5 pawns*
5 atom piece (Queen) = 8.5 pawns
6 atom piece (RNN) =10.5 pawns*
7 atom piece (Amazon) =12.5 pawns*
* Hypothetical Spielmann values
Pieces of Rook-strenght or less obey the equivalence 1 atom = 1.5 pawns,
but stronger pieces seem to gain additional value. I assume this is
because the strong pieces normally mask the weakesses of their components.
So rather than giving the BN a bonus for not being colorbound, a
colorbound 4-atom piece (Bishop-Camel for example) should be penalized.
Also note the each simple piece had one major weakness:
Knight: Short range
Bishop: Colorbound
Rook: only 1/4 of its moves are forward
So BN, RN and Queen all mask two major weakness.