You don't say what country that you are in so it's difficult to say how much it might be. In the US (from here) the price for replacing the 4S is $199 (plus $6.95 if shippng is required) :
Certain damage is ineligible for out-of-warranty service, including catastrophic damage, such as the device separating into multiple pieces, and inoperability caused by unauthorized modifications. However, an iPhone that has failed due to contact with liquid may be eligible for out-of-warranty service.
Here in the UK it's £159 (plus £7.44 if shipping is required).
(For info this is the Using iPad forum)