In a 36-page report to clients issued Monday, a Credit Suisse team led by Kulbinder Garcha took a close look at the iPhone’s price elasticity — Econ 101 jargon for the question: “If I lower the price of my widget, how many more will I sell?”

Garcha et al.’s focus is the iPhone 3GS, which Apple (AAPL) last month began offering to its partners for an ASP (average selling price) of $325, allowing the likes of AT&T (T), Vodafone (VOD) and Rogers (RCI) to give it to customers at subsidized prices ranging from $0.99 to $0.00.

It also marked Apple’s entry into what Credit Suisse believes is the sweet spot for mobile phone growth — the $250-$400 (to carrier) price range, where Apple’s competitors do most of their business.

How does this change the competitive landscape? According to Credit Suisse:

• Apple has an 85% share of the $500+ market and a 50% share of the $400+ market, but nothing to speak of, before the 3GS price cut, in the $250-$400 range.
• The $250-$400 slice of smartphone market is expected to grow 80% over the next four years, from 119 million to 213 million.
• Apple is well positioned to capture 25% of that market, putting more pressure on competitors whose margins are already being squeezed.
• HTC and Samsung are the most exposed, with a 22% and 20% share, respectively, of the $250-$400 market.
• The impact, according to this report, could be most severe on Research in Motion (RIMM) “given ongoing concerns around its product portfolio.”

On Monday, Credit Suisse lowered its price target for RIM from $30 to $20.

Its price target for Apple is $500. On Friday, when Garcha et al. wrote their report, Apple was trading at $374.94 with a 9.6 forward P/E they found “compelling” given the rate at which Apple’s earnings are growing. The stock closed Monday at $369.01.