Lately, for non-ladder, players will buy 40 PGems (not Amethysts, Rubies or Skulls) for 1 Ist rune.
For Amethysts, more like 12 per Ist. Skulls and Rubies 20 per Ist.
That’s for Non-ladder. Don’t know about Ladder. As for links, just look through the old trade postings, you’ll see deals at that rate all the time.