Apple's out of warranty or not covered under the warranty "repair" price pays for an exchange.
http://www.apple.com/support/iphone/service/faq/#warranty7
This 3rd party repair company charges $100 to replace the glass only - if the LCD is not damaged.
If the LCD is damaged also, they charge $200, which is the same as Apple's repair price that pays for an exchange.
http://www.drcellphone.com/iphone-3g-repair-service.php