If an spell cost mana, the amount of mana changes with the properties. The more properties, the more mana is needed to cast the spell.
If an spell cost health, the amount of health is always the same. I think it would be nice to increase the cost of health for spell gems when they had properties. Because Life Mages can easily acquire lots of healings so they doesn´t pay for maxed out spell while other Mages have to increase the mana of their creatures so they can cast spells with lots of properties.