So, a (difficulty) in 2001 chance of spawning a pigman per portal block per minute? If my math is correct, on the hardest difficulty using (say) 50 portals should produce 1 gold ingot and 4 nuggets in 1 hour.

3 possible values spawn a pigman -- 0,1, & 2

On average, 3/2001 = .0015 pigman will spawn per portal block per minute.

6 blocks in a complete portal, .0015*6 = .0090 pigman per minute per portal.

If the wiki values are correct, each pigman has a 50% chance of dropping a gold nugget, so 18 pigman are required for a gold ingot.

According to the wiki, there is a 2.5% chance of dropping a whole ingot, sword, or helmet, but I'm ignoring that chance for now because a) it's so small, and b) I don't know the chance of dropping an ingot (It could be 1/3 of all rare drops, but I don't want to assume anything)

50 portals * 60 minutes * .0090 pigmen = 26 pigmen per hour; leaving us with 1 gold ingot and 4 spare nuggets.

So yeah, this isn't the best strategy for grinding gold. . .

( RichardG: "reloading the map while falling makes you partially safe" -- what does 'partially safe' mean? )