Why he jumps on that part... well, it's a true-in-Taldor fact so perfectly safe so far as Maillol knows.
"5th circle is when you become able to Teleport, which is massively more in demand than most things someone can do at 4th circle. Also, I'm guessing it was Sevar who told you the figure of 100gp/week for her own earnings; she's right, but most fourth-circle wizards wouldn't make that. Not every 5th-circle wizard is qualified to be Security either."