Many may not know what is involved in developing a large system that is constantly changing, and evolving.
As a minimum, you will need the following.
A Project Manager.
A team lead.
In some cases, the project manager and team lead can be the same person.
A DBA to maintain the database system.
About 5 developers, one who is the expert on a source control system.
And in web development, a user interface person.
What will it cost you?
Keep in mind, that if this is a web project, you will also have hosting fees unless you host it internally.
Should you host it internally, you will also need office space.
As you can see, it is not a inexpensive endeavor, but it can be worthwhile if you have the product that people want.
Some recommended reading if you want to pursue your project further.