First things first, it's pretty easy to take a look into your analytics and identify the source of this traffic, try looking just by acquisition channel or source/medium in GA.

Secondly you need to ascertain the total organic potential and competition, based on the total volume available and competition you may have already hit a point close to saturation. Conversely there may be a huge opportunity that you haven't yet tapped into, making it very affordable.

The third thing is about risk mitigation, I'm always a big proponent on diversifying marketing and traffic sources as there will always be fluctuation, similar to diversifying your risk in stock portfolios, it's worthwhile looking into this as well.

Happy to discuss further on a call if you'd like advice on this from someone's who handled this on small up to multinational sites.

Cheers,
Mike