What Should be My Next Nikon Lens?

I'm abran, My uncle got me into Photography a year back, when I was a freshman in High School, And since then, I kept liking it. I First Used a Canon 600D with 18-55 kit lens, and Currently I'm using. A nikon d7100 with 18-140 VR lens. I bought this camera a few months back, I'm still learning though. I have saved up around $500 for a new lens ( It dosent have to be $500 exactly ) I mean I can't spend more than 500 on a lens. I like to shoot Landscape more, rather than portraits. Few of my friends suggested me to get the Nikon 35mm/50mm prime lens, But i'm not sure if it will be a good idea, because Mostly primes are for portraits. I do capture portraits too, but not always. Someone who has experience in Photography would be a little helpful to decide what kind of lens I should Get.

This is my album in my Facebook: https://www.facebook.com/...112690124/

To show what type of pictures I take.
*Please don't judge my photos, I'm just a beginner*

Photography

Landscapes thrive with a wide angle lens (12mm to 25mm). Look for a lens that's wider than 18mm (since that's the wide end of your kit lens). Avoid using lower than 12mm which is already fisheye (where the extreme distortion which you don't want for landscapes). With an APS-C body like yours, you should get dramatic landscapes with focal lengths from around 14mm to 18mm.

http://www.bhphotovideo.com/...DX_II.html

I suggest you buy one high quality fixed lens. You would be surprised how cheap used lenses are. I would get a good fixed telephoto lens, well in your price range