The flavour is in the bones not the meat. So you can save money and get just as much flavour using chicken racks. If you use 2 chicken racks instead of one whole chicken it will be stronger flavoured.
I use the bones from our roast chickens for stock - if there is stuffing I give it a good washing out first. I usually chuck the frame from a roast in the freezer and pull it out when needed. I make sure that any left over pan juices go in the cavity so that when it is turned into stock I get that flavour.
I use a bit of fish sauce in mine but then again I nearly always use my chicken stock in Asian style cooking. For Euopean cooking I would use sea salt.