cost of installation

We are going to build a home and (http://maplered.com) are considering a natural Stone (http://g603.net) floor. Something along the lines of Travertine, LimeStone (http://g603.net) ,FlagStone (http://g603.net) or Slate. We understand (http://maplered.com) that the cost to install and (http://maplered.com) seal this type of floor, is as much if not more than the cost of the Stone (http://g603.net) itself? Is this true or false?? Or, what is the ratio of cost to installation? Also, does the size of the pieces make a difference as to the installation cost?? The larger pieces costing more or less? Or if you use different size pieces throughout?? We are in the Phoenix area (where unfortunately for us, building is booming) Please enlighten us.
Thanks a lot
s.enniss