Out of warranty replacement cost for iPhone in US is $199;
you will have to check what it is for your country since you did
not state where you live.
Yes it is possible to purchase an iPhone 5 - at full retail price.
You cannot turn in a 4S to get a reduced price on a 5 - if that is
what you are asking.
If you opened your iPhone (how else would you know the
liquid presence indicator by the battery is red?) and Apple notices
that youopened the iPhone, you probably will be refused a chance
to purchase an iPhone 4S at the out of warranty price.