IIRC cobble is worth 60 each, an igneus makes one cobble every two seconds, and it takes 5600 or so per iron block. I'm tired as heck, so I might have something wrong with any part of this, but 5600 / 60 * 2 is one iron block every 186ish seconds per igneus, and there's really no limit to how many igneus's you can have pumping into one nexus.
As always, feel free to correct me on any part of this, these numbers were off the top of my head, but I set this system up last week. I think the Ars Magica version was the one previous to the current version, though. So it might have changed.