Not really worth it.

It just comes down that we can’t give everything away for free. Ninja Forms does a lot that most other free plugins don’t. We have to decide what will be free and what we will charge for. Unfortunately not everyone will agree with us on where we draw that line.

I would also like to add that Ninja Forms can be easily styled by following the documentation here: https://docs.ninjaforms.com/article/239-how-can-i-style-my-ninja-forms