Try this...

SELECT ISNULL(min(BudgetId),

       ISNULL((SELECT max(BudgetId) + 2

               FROM tblBudgetYear), 2) - 1

FROM tblBudgetYear t1

WHERE NOT EXISTS

  (SELECT 1 FROM tblBudgetYear t2

   WHERE t2.BudgetId = t1.BudgetId - 1)

 AND t1.BudgetId > 1