As the chart below indicates, there’s quite a range, from Piper Jaffray’s bullish 45 million to Bernstein’s bearish 10 million. All estimates are for calendar year 2009 except Lehman Brothers’, which is fiscal (i.e. Oct. 08-Sept. 09). (E-mail subscribers can see the chart here.)

The reasoning behind the new estimates is as simple as Econ 101: as the price goes down, sales go up. “Apple is going for volume with the iPhone,” writes Charlie Wolfe at Needham. “iPhone sales should increase dramatically in the second half of the year and beyond.”

But as Piper Jaffray’s Munster notes, “In reducing the entry-level price of the iPhone to $199 and eliminating revenue sharing agreements with its carrier partners (using one-time subsidies instead), Apple is clearly making a trade-off.” Like most analysts surveyed, he believes it’s a strategy that will pay off in the long run. (See The iPhone’s new business model.)

Meanwhile, AppleInsider has published a chart that suggests how much work Apple (AAPL) has ahead of it. Combining sales numbers from its quarterly reports with Steve Jobs’ announcement that as of Monday Apple had sold a total of 6 million iPhones, Charles Jade came up with a bar chart that shows a sharp fall-off in June quarter sales:

There are still two and a half more weeks in the quarter, of course, but given that Apple has run out of product — and tipped its hand about what’s coming in July — the only iPhones likely to be sold in June are first-generation models that folks manage to unload on eBay.