What affects the cost of damp proofing
The cost of damp proofing depends on two things: the total length of wall affected and how deep the moisture has penetrated.
Rising damp doesn't hit one spot — it tracks along the base of every wall in contact with the ground. A single external wall is a different job to a home where damp has crept through every ground-floor wall, internal and external. The more linear metres of wall that need a new damp-proof course, the higher the cost of damp proofing.
Older homes — particularly pre-1960s homes across inner Sydney and Melbourne — often have no damp-proof course at all, or one that's worn out after 60+ years of doing its job. The cost of damp proofing these homes sits toward the higher end because every ground-floor wall usually needs treating.
The part most homeowners don't factor into the cost of damp proofing: the damage you can't see. Salt crystallising inside the masonry doesn't show up until the bricks start crumbling. Treating it early is the cheaper option — not because of urgency, but because less wall needs treating.