A conversion per click ratio would depend on what keywords you are using, the PPCs you are using those keywords on, and an infinite number of other variables. Terms with high traffic (e.g. travel) are going to get fewer conversions in general because people are searching for a broader scope of information as opposed to clicks on "long-tail" terms where peoples aim is more focused (e.g. cheap airfare to houston). The PPCs you are using as well as their partners will also play a role - primarily because users get to your site not just through search, but through contextual advertising and a myriad of lesser used practices (i.e. domain parking). It's virtually impossible to say "this is my click to conversion" ratio as all will be different.