You could make the case that Masa overpaid for the stock based on current operations... But then again, the spectrum Sprint has (now combined with Clearwire) could mean otherwise. I just remember when as a Sprint shareholder during the acquisition, I was offered a choice of cash or new Sprint stock. I opted for cash... And so did many others... In fact, too many... So the cash buyout SoftBank provided wasn't sufficient and people like me wound up getting a mix of cash and new Sprint stock.
See here: http://www.bloomberg.com/news/articles/2013-07-08/holders-of-97-of-sprint-to-get-cash-in-softbank-takeover
Maybe that burned a hole in the pocket... Or perhaps Sprint took more funds after the acquisition than Masa expected given how much fixing needed to be done.