Although the iPhone 4S retails for around $650 (16GB model), most folks can pick one up for significantly less than that. Carriers will generally absorb a large amount of the handset's cost as long as you promise to be a long-term customer.
Here in the US, all of the major carriers offer the 16GB iPhone 4S for $199 along with a two-year contract. And C-Spire Wireless, a southeastern regional operator, offers the entry-level iPhone 4S for just $149...