you can get the cases on ebay for $30 or so..
you can get the keys on ebay for $50 - $75
here are a few links:
eBay Motors: 2005-2006-2007 CHRYSLER 300+300C Remote Key Shell/Case! (item 150294536728 end time Sep-21-08 0242 PDT)
eBay Motors: 2005 2006 2007 Chrysler 300C 300 Remote key 56040652AD (item 370087017269 end time Sep-16-08 1605 PDT)
do some searching on ebay..
make sure it is a CHARGER or 300C key, Magnum Keys do not have the trunk button.
you can get the replacement case and put your guts in it, that should work just fine.. Home Depot will cut the key for you or any lock smith for $5 or so..
now, adding a key..
in order to add a key you need either 2 original keys (to do it yourself)
or a dealer Star Scan and your Vehicle PIN number (only dealer can get)
most dealers (MOST) will program the keys if pre-cut as a courtesy, some will charge you an hours labor $75 -$100
I have found that Dodge dealers are a lot friendlier with regards to stuff like this..
also I would advise that you get one of the VALET keys..
store it somewhere safe, they if your fobs ever have issues you always have a second key on hand to program them yourself..
personally I have 4 keys 2 VALET and 2 Fobs so that I can program them if/when needed..
I also own a star scan now so if you are local to southern california, get your PIN from your dealer (they have to give it to you if you ask) and come see me, I can program them for you, no charge.