Hi,
You do NOT have to pay anything - you have already paid for the licence to use the DVD, but seeing that its broken, you can get the ISO to replace it, install it and activate with your licence key that you got with your DVD. Nothing more to pay.
When you paid for your Ultimate, you actually paid for the licence key - the media (DVD) is a fraction of the cost. In fact, if you have a friend with exactly the same version of Windows 7 on a DVD, you could have installed with that, using your licence key to activate it.
Regards,
Golden