Usually, developers make a quick estimate of the order value in advance. You can get it in a couple of days, usually for free. If the cost suits the customer, he signs a contract, which calculates the exact price of the application development, the deadline for the completion of the work and a description of the final result. Sometimes the price can change if during the development process it was decided to add additional functionality. But this is agreed with the customer. By the way, it will be useful for you to read the article How much does it cost to hire developers and find a more detailed answer to your question. Separately, I would like to note that the average cost of developing a mobile application depends on the complexity of the application.
