What is the moderate dihedral angle for a low wing airplane?
Please do not worry about the loss in lift. It is real, but so small that it can be neglected. A poor flap or aileron seal will cost you more in L/D than the dihedral ever will.

The loss is proportional to the difference of the cosine of the dihedral angle to unity. With 5° (which is on the upper end of typical values for low wing aircraft) this is just 0.38%.

The exact amount of dihedral depends a lot on details of the configuration:


Aspect ratio of the wing
Fuselage size relative to wing root chord
Wing sweep
Directional stability


In general, you need a good balance between $c_{l\beta}$ (dihedral effect) and $c_{n\beta}$ (weathervane effect).