Depending on which discount rate you use in your NPV calculation, it's a good deal less than $240. Also, if softbank are like mobile providers in Europe, it's a $10 credit on the most expensive plan with everything and the kitchen sink included - not the smaller tier or pay as you go plan that's actually plenty for most people.

Also, $10/month (well, less, depending on what the iPhone is actually worth) doesn't seem to be totally out of whack for a short term promotion to grab some market share.