Edited by: Kethas Protagonist on 23/08/2011 23:03:32
Blake's linked site is correct. I'm adding to what he said, not correcting him.
All NPC-sold items have a "base price." You can find this in the patch-specific static datadump files that CCP provides. The price that NPC sellers actually charge varies in a small window around that - they lower their price if something doesn't sell much, and raise it if it does.
People buy blueprints very rarely, so almost all blueprint-selling NPC corps (possibly all, but at least every blueprint seller I've ever seen) lets their price decay down to its minimum, which is 90% of the base price. You can thus find the NPC price of a blueprint by multiplying its base price by 0.9.http://zofu.no-ip.de/bpo
can be used to find which NPC corps sell a given blueprint (and is a great resource for blueprint calculations and info in general). So, for example, searching for Capital Armor Plates Blueprint, we see that the base price is roughly 1.291B and it's sold (only) by the Amarr Navy corp.