I asked ATI the same question a few months back. I think it was around $450.00 per channel. I'll have to see if I can find the thread here. It may have been more than a few months ago. But I'm thinking it was $450.00 a channel + shipping.
I wonder if that means all models lower than the 5 and 7 channel have the same transformer and other "stuff" to scale up with just the amp boards.
I thought I had read somewhere that ATI told someone who wanted to upgrade from 2-5 that it would need to get sent in for a transformer swap, but I don't recall where I read that or if I just fabricated it.