I'd have just gone with an amount of tria... and yes... as grief to the pickpocketed player.
(only as example...)
Lvl 1 = ~10 tria if successful
Lvl 2 = ~20
Lvl 3 = ~30
Lvl 4 = ~40
Lvl 5 = ~50
Lvl 6 = ~75
Lvl 7 = ~100
Lvl 8 = ~200
Lvl 9 = ~300
Lvl 10 = ~500
Lvl 11 ~ 800 tria
Lvl 12 ~ 1200 tria
Lvl 13 ~ 1600 tria
Lvl 14 ~ 2000 tria
Lvl 15 ~ 2500 tria
(and if any of these exceed 5% of what a player has in total.. then only 5% should be taken [stops new players getting completely robbed of the few trias they have!!])
