How about the cleric's base speed increases by 5 ft. for every 5 levels for a total of +25 ft. at 20th level? This would mean +5 ft. at 1st, 5th, 10th, 15th, and 20th. If that's too much you could always do +5 at 1st, 7th, 13th, and 19th for a total of +20 ft.