Probably you are ranking because of your site is 6 years old and visitors must be clicking the other pages of your site. The reason your Cpc is high because google pays more when the traffic is coming from google itself and its the best quality you can ever have. I have seen this on a lot of sites. For example there are 2 sites in same niche when traffic comes from fb,twitter etc. your cpc will be much lower than the site with traffic coming from google. What i mean is the traffic source is the most important thing imo after keyword selection.