I started the day with a PP skill of 80. I went around Whiterun and stole some coin from the Companions, then some from Riften. My PP skill is now at 95 and I was only doing this for a few minutes. It seemed like every time I pickpocketed, the skill level went up by 2 or 3. I even managed to level up my character from 41 to 44 by doing this today.

And the stuff I was stealing wasn't even that valuable. I only managed to steal maybe 8k total in coin.

It levels that fast.

This has always been a flaw in TES' skill-leveling system in that some skills level fast as hell (Merchantile in Morrowind, Alchemy in Oblivion, Sneak/Pickpocketing in Skyrim) while others level ridiculously slow (Speechcraft in Morrowind (iirc?), Merchantile/Destruction in Oblivion, Destruction/Restoration/Speech in Skyrim).