First a disclaimer - I have a lot of experience designing/developing large web apps - less with mobile apps but similar principals apply for sure.
The service you need first is someone to help you to define the functions your app is going to perform. So I'm not talking detailed screens and I'm not talking detailed tech questions. But I am suggesting you work with someone to get very clear on how the functionality of your app relates to your business plan. ie what features are most important on launch, which ones you are planning to develop soon after.
This is a consultant you're looking for although plenty of developers/designers will be able to do this job quite effectively if they also have business experience.
Once this information is clear then you would probably want to contact a developer for rough estimates on development costs/timeframes for each module you are building. They'll make decisions about which pre-built components to use in your project. Then you can weigh the estimates for each function against their projected financial value.
With this information you can get the UX guy to create detailed schemas for how the first release of the app will look from the outside. And you won't be wasting any time/money because the broad areas you're working on are already costed approximately with the developer. Then you can take the UX guy's output and send it to the developer to get much more precise estimates and you can make your final choices before pressing 'go'...
You will probably find that from a development perspective there will also be 'known unknowns' - no matter how skilled/experienced the developer they will not necessarily know off hand how long tasks will take. So often you'll need to pay them to explore these areas to get a proper costing. It depends how big/complicated your app is. Sometimes the developer will be keen to learn or to impress you and get your business so some of this they will absorb themselves.
Lastly, when the UX guy has done his job, make sure you get from your developer a proper costing for each module of the app. So we're talking something like:
First Screen - 300usd
API connection for 3rd party integration - 800usd
Video display page - 800usd
...and maybe even a break-down within each section.
Then make sure to get your developer to define a timeline with verifiable milestones for payment. So there may need to be advance deposits at some points depending on how your developer's business is structured but make sure that there are points where you can say 'when I can do X and Y and see for myself that it works' then 'payment is released'.
Sometimes the budget and the timeframe/milestones data will need to be revised but at least this can be a collaborative process between client and contractor. The better you communicate the more successful your working relationship will be.
With the best will in the world there are still often budget overruns but you can minimize these with the practical systems I've defined above and also by being as unambitious as possible with the technology within the bounds of what your business demands. So you can be as ambitious as you like with the business but always build the minimum sized tech solution you can because I guarantee you, the day your project hits the public, your plans will change and you'll be glad you've got time and money left over to respond appropriately:-)
I hope that helps, if you have further questions I'd be happy to help.