Write SQL query to solve given problem: What is the percentage difference of River City sale compare to Frostie?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST((SUM(CASE WHEN T3.BrandName = 'River City' THEN T2.PurchasePrice ELSE 0 END) - SUM(CASE WHEN T3.BrandName = 'Frostie' THEN T2.PurchasePrice ELSE 0 END)) AS REAL) * 100 / SUM(CASE WHEN T3.BrandName = 'Frostie' THEN T2.PurchasePrice ELSE 0 END) FROM rootbeer AS T1 INNER JOIN `transaction` AS T2 ON T1.RootBeerID = T2.RootBeerID INNER JOIN rootbeerbrand AS T3 ON T1.BrandID = T3.BrandID