Wizard quests have always been profitable for me. He never asks for expensive items. Now today I haven't finished them all yet and I have had to skip 2 because he asked for an item which was over 90k and one over 40k. He has never asked for items above 20k. Why is this?

It's because restocking has been completely screwed up for weeks+ now. Many items haven't been restocking so the prices are really inflated.

The items that he's asking for could also be fairly new items. I've had that a few time, he once asked me for an item that was so new that there weren't any available in the SS at all.