Extending the Senior Citizens Rent Increase Exemption program to people with disabilities could cost a little more than the Independent Budget Office initially estimated. Since City Limits reported that the annual cost for the extension by the fifth year could run $4.3 million [“Stopped Payment,” July 30], the IBO has reassessed its figures, and found that given the cumulative impact of a rent exemption for seniors and New Yorkers with disabilities with incomes up to $50,000, as has been proposed, the program extension could cost as much as $17.5 million by year five. Added administrative costs could tack on another $1.3 million, said the IBO.