I can refer you to the company owlab. We ordered a mobile app development https://owlab.group/services/mobile-development from this company. They have a very experienced staff, they always know how to develop mobile applications correctly. All because of their huge experience. The architecture of the app is always built correctly. The technologies are right, they don't use anything unnecessary which can slow down the app. The cost of developing a mobile app is always different, it all depends on the task.