I don't have a ton of experience in this space, but I did go down the road of almost launching an app. The budget you need to launch a successful app right now is enormous. Exact numbers that you need to budget would depend on what exactly you are trying to do. The cheapest way to get an app launched right now is to do a web based application, especially if the app does not need specific hardware to run. Having a web based app eliminates the need to build an app specifically for IOS and Android. The cost all depends on how many hours the developers have to put in for the app, how many pages, and how difficult the project will be. You also have to figure in costs to retain a developer to fix bugs and push updates to the app.

The easiest way for you to do this, especially that you are targeting millennials, is to use social media advertising linking to a free ebook or webinar about something relevant to your newsletter. The key here though is to require an email opt-in to your newsletter. You'll also have to use some good click bait on your advertising with a title like "Millennials not doing this one SIMPLE thing are missing out on thousands of dollars". You can build a website with an email opt in pretty easy on clickfunnels. You should also start building a brand up on facebook, youtube, and instagram to start getting some organic growth without paying for ads