Given that iPad app pricing has been substantially higher than it is for the iPhone, I can't imagine too many iPad buyers balking at a reasonable OS upgrade fee

Given that iPad app pricing has been substantially higher than it is for the iPhone, I can't imagine too many iPad buyers balking at a reasonable OS upgrade fee. Here I define "reasonable" loosely as "cheaper than a desktop OS upgrade", which would put the price cap somewhere in the $20-25 range.