In the past four years, Apple iPhone's normally are manufactured for two years, one year as Apple's top of the line offering. After the release of a new model, it slides down as the second Apple iPhone for a year or so.
This looks like it will change in 2011. When the iPhone 5 comes out, the iPhone 4 and 3GS will continue in production. It looks like Apple will be keeping three iPhones in production to allow them to compete better against Android. The iPhone 3GS will be offered contract free for a price of US$350, reports CNET. That should mean about Php16,000 to Php17,000.