I'ld like to use DNN as a base to build my own closed source product. Question is:

- Does the license permit me to do so?

- Do I have to pay, how much and how do I pay?

- Also am I able to rebrand?

Thanks in advance for your answers.

From a purely non-license perspective (since Sebastian already answered that portion), you need to ask yourself if it is really in your best interest to branch the code into your own product and take it from there.  Doing so will make it extremely difficult to keep up to date with core changes (ie., everytime a new core comes out now you will have to either peruse their code to find the changes, or reintroduce your own code).

Depending on your business goals, you might be much better served by creating extensions by use of SkinObjects and Modules.  This allows you to keep the core the same, can upgrade with core upgrades with the least resistance possible, and still benefit from the entire community as a whole. 

There is nothing stopping you from creating a "vertical market" with DNN through your own modules that target a specific niche market (which perhaps is your ultimate goal?).

Good luck whichever route you decide to take.

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