FIREBALL costs: we all know this classic spell of elemental destruction. It’s been the devil’s right hand of most every battle mage for 40 years now, but though it’s been great for chewing through tightly knit bands of baddies, the destruction meted out by this spell has always been muted. This may be more mechanical than narrative, but I think one of the simplest ways to increase the cost of this spell is make it destroy loot. Another obvious cost would be the extreme fear that a fire wizard would strike in the hearts of people for whom fire poses a very real threat to the existence of entire cites. You could expect the regulations encountered by such people would be draconian in the extreme.
Another great cost for such a high energy spell would be body heat. After casting this spell, the caster is cold all the time - like without fire near or a coat on, could freeze to death in summertime. That, or they need to be in contact with some store of energy, such as turning a woodpile into ashes, or turning a great sack of coal into dust. You have to be careful with that sort of thing, though. Players will try to weaponize the cost, if they can inflict it upon others.
FIREBALL taboos: I’d say places of desolation, or the metaphysical desolation of the caster, would be incompatible with such an energetic and resource hungry spell. A depressed wizard is unlikely to be able to muster the energy for an explosion. It may be also that a supremely cold place would at the least be difficult to summon a fireball into. Again, the fiction of the origin of the fire becomes very important in explaining what sort of place may be hard to cast in. If the caster is opening a portal to the plane of fire, or the surface of the sun, or what have you, then places planarly locked would block the spell, but if it is through accessing deep seated rage, or using the stored chemical energy in wood, different circumstances explain the taboo.