Friday Talk: How much does it cost to develop a mobile application?

Well, it depends on the complexity of your project. But, if you create an app from scratch or with an agency, it can cost between 10K to 30K. In this price, I include the time to design and to developp the app. And also the time needed to manage the project.
This is what an agency will charge you.

If you create native apps, the development for each platform is different. Sometimes you can re-use the same design, but, usually, even there, you'll have to adapt it to the look and feel of the targeted plateform.

When you use an app builder, you can cut the price by 100. This is possible since thousands of people use the app builder and the small amont they pay to use it is used to develop the product.
The work is done once for everybody.

The approach is different. An agency will do the work just for you. They will start from your needs and ... tailor a developement for you. 
With an app builder, you will do it by yourself. Using an app builder is faster and cheaper. But your needs have to fit with the features provided by the app builder. App builders offer the possibility to create plug-ins, and this can help you if you want to add in your add something higly customized and not provided by the product. 

In both case, you'll get assistance. 

Reagrding your app updates, with an agency you'll need to pay for each update you need. Because (again),  people will do it just for you and a team will work on your needs. With an app builder, and specially with GoodBarber, upadtes are included in your subscription, new features as well. 
A team is working all the time on the product.

Yes, exactly.