Magnetizing your models is a great way to get the most out of each kit. You can swap weapons to totally change the way a model plays. This helps to keep things fresh and to make adjustments after the army is painted without having to get an entirely new model painted.
I charge a flat rate of 1 GBP ($1,35) per magnet used, which covers material and labour (painting additional arm/weapon not included). Please remember that you will typically need to consider the mount magnet and the magnet on each optional piece. For example, to magnetize a dreadnought and two weapons would cost 3 GBP, 1 GBP for the mount and 1 GBP for each weapon.