Knowledge source:- I'm an app developer myself (heading app development company www.agicent.com.
I believe you understand that an App like OpenTable basically has 3 major parts;
1. one is the end user App that you see on the front end on iOS or Android, (native)
2. Another is an App or a web portal for restaurant owners to receive and confirm bookings, see orders etc.
3. and a Backend that you don't see (which has the DB, APIs, business logic, Admin panel, reports etc).
So broadly breaking up these modules, I can do this project as follows:-
1. Minimum 60 to 80 hours for all UI design.
2. 350 hours for iPhone App for end user
3. 250 hours for An app (iPhone or android or web) for the restro owners.
4. Minimum 400 hours for the backend (DB design, API, admin panel with basic features).
5. An overall 150 hours of QA for each module.
6. Take a buffer of 10 % minimum.
These hours also include the research and planning on technology, third party libraries to use, and interim releases (at least 10 for both apps and admin etc) for your testing.
To get this done you need to have a great tech. architect cum supervisor who shall oversee and decide on everything, very good developers for iOS and Android (at least 1 senior + 1 assistant each), and a DB programmer and one Web developer (for admin panel), and a critical QA.
The actual release time of this whole thing could be:-
1. 3 months from scratch to one App (end user) and backend (without Admin panel).
2. Another 1 month for restro owners' app (or site)
3. Another 1 month for admin panel release and to finish everything.
While an offshore app development agency like mine would charge somewhere around $ 20-25 K for this project, the same in the USA would charge you 5-6 times, while the same in freelancers world could be done in like even less than 5 K (but that won't be what opentable is).
Feel free to ask more, I'm happy to assist.
PS:- some relevant apps from my portfolio
• Food ordering Apps - https://goo.gl/lb8Tfg , https://goo.gl/WTzXy2
• Limosine (Cab) Booking App (For a startup in Boston area, USA)- https://goo.gl/iIi2qs
And couple of more I'm getting done right now and wouldn't mind demo that.