Betty, along with bishop Patra, bandit Lyla and Pirate Morena gain skill called Gold Get on awakening. It grants them a 10% chance to get 100g per unit they kill. Betty is the cheapest, easiest to get and Field of those. Pastorial gate, along with few others map I forgot the names of, has the best ratio of mobs on the map to stamina, so as a result you sometimes get more money from it than for example from dragon hunting done by sorite.