The biggest cost ahead for large mortgage servicers may not be “robosigning” settlements or buying back bad debt – it’s the follow-on mortgage products like home-equity loans that take longer to go sour. A report on Monday by CreditSights is the latest sign that the biggest cost to banks from the mortgage crisis could be home-equity loans – whose credit-card-like aspects tend to keep borrowers current long after they’ve maxed out the first mortgage. CreditSights estimates that Wells Fargo has the most exposure to home-equity costs, at $7.8 billion. JPMorgan Chase is right behind with $7.2 billion, followed by Bank of America at $4.9 billion and Citigroup at $3.6 billion. However, the expected lag in performance has allowed big servicers to prepare for the coming HELOC write-offs.