The would get most of that from the continued subscriptions of current customers who didn't get up and leave for another carrier just to get an iPhone. And Sprint knows exactly how many subscribers have been leaving them with each new iPhone introduction, and with each contract expiration. They may have to do almost anything ($20B in debt?) to put a tourniquet on the bleeding.
Like stated, even if they ended up with +30mn customers, they would also need to rake in (an additional) 1k in profits (relative the alternative) from each customer. Unless you have extremely high margins, and really high phone bills, in the states, well... i just dont see how they would ever break that one even.
And, even if they could, money could definitely be better spent elsewhere.