In general they reduce costs by using their knowledge and software for multiple customers. But they have more overhead costs (like marketing).
And, of course, different companies have different knowledge, different experience, different costs and different price.
There are so many companies that it's impossible to name the top ones. But I think that Microsoft (with Office Mobile and other business apps) surely is one of them.
I need to know how to any top mobile app development company reduces the cost of mobile app development and what are the top companies for mobile app development?