The insulation materials themselves are often very cost-effective materials.
Costs are incurred mainly due to implementation, and due to things like sub-constructions or exterior cladding, setting up of scaffolding etc. That is why it is so important to do everything properly while you're at it
3) : the principle
“If it's worth doing at all, then it's worth doing it well!” applies here, because a little more insulation material only costs little, but doing everything all over again would be really costly.