Cost to develop an app?

Well, it obviously depends on the project

Something you might want to keep in mind, the building is never really done. A lot of times, development starts only once you get customers to use it. Based on their feedback, you might make minor modifications or have to revamp the product completely.

It really depends on the complexity of the project, on where the developer is based and therefore their hourly rate. A tip I feel like giving you as I'm also a non-technical founder, make sure to speak to the developer only once you have a precise idea of how the product will work, on my first project I found myself changing ideas back and forth after the development started (mostly on little things) and it only led to a waste of time and money. Good luck!

It is impossible to answer this question with so little information.

Could you at least describe the main idea of the app or some of its core functions? You can also send it to me privately.

There's so many factors that go into this. Consider:

1. Is language/time zone/geographic location of the developer important to you?
2. Are you looking for high-level expertise, or to take a chance on someone less experienced (and therefore cheaper but riskier)?
3. Are you looking for a quick prototype? Or a full app?
4. Are you budgeting for new features users will request, tweaks you'll want, bugs that will need fixing, 3rd party services that will change things on you, etc.?
5. Since you're a designer... Does the developer need to make everything pixel-perfect? What about mobile responsive? What about animations?

I'm a freelance/consultant web developer, and depending on how you answer the questions above (plus others), the quote I would give you could be anywhere across the entire range you've specified.

Generally, though, I think the most important thing to recognize is that software is malleable. Especially for a very open-ended project like you're describing, the price is really a function of how much you want to build - and when you want to stop building.

Ohhh, it really depends. I am an iOS dev (freelance) with about 8 years old experience, and I have had projects for 4k $ and 50k+$ for the iOS apps.
It is all about size and complexity. Btw. If you will need some help with technical project estimation, feel free to ping me.

Anything between $10.000-$10.000.000

Joking aside, you'll have to make a plan on what you want to build. Then discuss the plan with an agency or someone technical and you'll get a rough estimation on the complexity/time needed.