What are the effects of dihedral on net lift of a wing?
The effect of dihedral on lift can be obtained applying high school trigonometry:

$$
L=L_0 \, cos \,\theta
$$

where $L_0$ is the lift of the original flat wing and $\theta$ is the dihedral angle. This holds true for negative values of $\theta$ as well.

Since the cosine can be approximated by unity for small angles, the net effect is negligible in most practical cases, e.g. $ cos(5^o)= 0.996$