Not to be sneezed at means that the rates are not to be ignored, but here in context the rates for night time wardrobe assisting were easily ignored by the girl 
Consider the statement "The rates for night time wardrobe assisting were something to be easily ignored or passed over by a girl with neither influential contacts.". Is the statement "The rates for night time wardrobe assisting were not to be sneezed at by a girl with neither influential contacts." false? Yes/no/unknown