We recently updated our ads with ad extensions. Our costs went way up but our conversions haven't. We can't pin point why but it's mainly the sitelinks extensions that are so high. When we look at the sitelinks extensions it shows a Total - all sitelinks line but it's only totaling the first sitelink extension and not all four.
When you say costs, which costs are you referring to? What kind of bidding and budget options are you using? Ad extensions alone are definitely not all there is to it.
The total line represents all clicks on any particular link. There can be anywhere from one to four individual links shown, but you can have more than four individual links. So in your case, the "first" individual link, if sorting by clicks, was shown every time any individual link was clicked. The rest of the individual links may not have been shown every time any sitelink got a click.