At $30 a month for required data plan, that'd be $360 a year new revenue (which doesn't really "cost" VZW anything since they have the capacity), which gets you pretty close to the $600 retail price. Anyway, VZW's most profitable customers would be ones who DON'T take advantage of the subsidized phone every 2 years.
From a stock valuation standpoint, you only care about the growth in # of customers. Not really any significant impact to extending people (unless they are upgrading to more expensive plans) since you assume a good business will retain those customers, anyway. The revenue growth opportunity is in other services (such as Vcast) where VZW, like all carriers, has failed rather miserably. But with LTE & FIOS there may be some interesting opportunities in the future.