Apple, which will launch its new iPhone 3G S handset in eight countries on Friday, is expected to sell half a million of the devices before the weekend is over and sales of the $99 iPhone 3G model are tallied into the mix.

The forecast comes by way of Piper Jaffray analyst Gene Munster, who notes the half-million mark would compare to sales of 1 million iPhone 3G handsets during its inaugural weekend last July, and 270,000 original iPhones back in June of 2007.

“Several factors lead us to believe Apple will sell less iPhone 3G S’s at launch than it did for the iPhone 3G including a less dramatic change in value proposition and fewer available countries at launch,” Munster wrote in a research note Thursday.

More specifically, the analyst points out that the launch of the iPhone 3G last year came alongside a 50% price reduction from the original models, which drove sharp increases in demand.

With most customers being able to purchase the new iPhone 3G S at identical pricing, the change in value proposition for the new model is therefore not as meaningful as last year, he said, which should likely translate to fewer units sold at launch.