Not necessarily cheaper... Bear in mind, the cost of buying pocket would include buying pockets infrastructure, as well as paying them a premium for their clients. It would almost definitely cost more, and they'd still have upgrading to do.
I'm not saying it's a bad idea, because obviously we don't know the specifics of what buying pocket would entail, but it's probably just as easy for cricket to build their network the way they want, knowing they'll probably end up stealing most of pocket's customers anyway.... And once cricket's infrastructure is in place, and pocket starts floundering, cricket can buy what's left of pocket for pennies.