ismant,
The repair of an iPhone for anything that is not covered by the warranty is done for a flat rate repair. Whether it is accidental damage, abuse, cosmetic, or the warranty has expired.
The cost is $199 + $6.95 for shipping for the 4 GB, and $249 + $6.95 for the 8GB. Batteries are $79 + $6.95. This is discussed in the iPhone Service FAQ which can be read here:
http://www.apple.com/support/iphone/service/faq/
If it is something that is covered under the warranty, or AppleCare Protection Plan then there is no charge, unless you choose the option to use a loaner phone while it is in for repair.
Hope this helps,
Nathan C.