New to slow motion cinematography.

Hello,

I have been doing photography for the last 2-3 years and have been having fun with it but since October last year I have owned a iPhone 5s and have been really enjoying the 'slow-mo' feature the camera offers and now I'm thinking about getting a proper slow motion camera. From the videos I have recorded on my iPhone the majority of them have been gigs and music festivals with the crowds dancing and jumping around. I want to continue doing those kinds of videos but to a higher standards and with more FPS, thinking roughly 500 FPS?

My question is, can someone help advice me with the cameras which offer the best bang for my buck? And is also best suited for the kind of videos I have been recording so far? Any help will be much appreciated.

Thanks in advance, Matt