I need to know how many credits an AT-AT costs?
I've searched but I can't find any resources or materials that tell me how much it costs.
The Age of Rebellion Core Rulebook identifies the nominal purchase price of an AT-AT as (approx) 150,000 Imperial credits.
Given that they're described in other rulebooks as being non-available to anyone outside the Imperial Military, I would argue that they're not really purchasable openly, regardless of the size of your pocket-book and even as military surplus. You'd either have to get one on the black market or pay someone to steal one.