If you’re the kind of investor who likes to fiddle with Wall Street’s assumptions, you’ll get a kick of the report the folks at Trefis issued Friday.

They started with their version of Apple’s (AAPL) business model and looked at what it would take to lift the stock from Trefis’ current price target ($400 a share) to the one the Street has started to whisper about: $500.

This is easy to do with their Apple model, which like the 100 or so other company models Trefis maintains is fully interactive and fun to play with.

To get to $500, according to Trefis, two things would have to happen:

• The iPhone’s market share would have to grow significantly. To take the target to $400, Trefis had already assumed Apple’s share of the worldwide mobile market would grow from 4% to 14%. Going to 17% would add $55.
• The iPad would have to get even hotter than it is. Using iSuppli’s estimates, Trefis assumed that Apple would ship 12.8, 43.7 and 63.3 million iPads in 2010, 2011 and 2012, respectively. If Apple can reach 90 million units by 2012, the stock price should grow another $45.

$400 + $55 + $45 = $500

For the full report, click here.

See also:

• How much is AAPL worth? (Hint: Trefis’ answer in April was $267)

[Follow Philip Elmer-DeWitt on Twitter @philiped]