classified adds web site- pay per click sponsored links

I own a classified adds website running a PHP Script( [login to view URL] ). I would like to have implemented a pay per click sponsored links system similar to the one on gumtree.com. A number of adds (links) on each page depending of the content of the page. Gumtree does that with Google Adwords(Adsense)but I would like to do it a bit different. To have this as an option to make your add featured but in stead of paying per days to pay per clicks(and in the same time to keep the pay per day featured adds option). I would like to have a limited number of sponsored links on each page(3-5) on the head and bottom of the page. How long would somethilg like this take and how much would it cost?