It's never easy to understand how much something will cost because there are so many moving parts, phases, non-technical costs, and degrees of completion that need to be considered.

My advice to clients is usually to take discussion to a higher level and consider things like:

1) Do you want to replicate growthhacker.tv exactly? Or are you just wanting to make a business similar to it, and you want to get to an MVP with similar functionality? They have probably invested a lot of time/money into refining the application with promo codes, analytics, and lots of admin things that you can't really see. Do you want all of that, i.e. the cost to build a mature business, or are you just interested in what it would take to build the core technical system?

2) A lot of your cost will hinge upon your experience and what you can bring to the table. Do you have any experience with project management, design, ui/ux, etc? What level of service do you need?

3) It's also a good idea to look at this in reverse. Instead of 'how much will it cost?' you can consider 'how much do you have to invest?' - then get an idea of what is possible with your current resources.

A good technical person/consultant should be able to describe some ball-park prices based on various scenarios so you can get some context. There will really never be a 'cost' for something like this, but you should be able to get a sense of what possibilities exist and the general investments required for each.

Good luck!