Price is also dependent on the motivation of the buyer and seller. A bike is basically worth what the buyer is willing to pay. A gotta-have-it-now buyer might give $4000 for that 2013, while a buyer just browsing might offer $2000 for the same bike. Also a factor is supply and demand. Is the market flooded with used NC700Xs or are they rare and in demand in your area? There are just too many variables to determine a price, but NADA and other services can give you historical average sale prices and offer some guidance.
To answer the original question, I’d offer $1500 for a stock 2013, with 31,000 miles, less if it’s grey instead of red. Why so low? Bikes are cheaper in the Midwest than in California. Plus, I don’t need or want a 2013 NC700X right now. So is that much help? Probably not.