Well... I have never played Morrowind, so I have no idea if 200 coins is strict or not, but it does not sound like a tough penalty.
Losing possessions sounds firmer to me, i.e., losing the tools of PKing or unique items, having the illicitly-gained spoils seized certainly makes sense in an in-game social context, once perpetrator is caught.
You could set the penalty as an increasing fraction of your goods/coins (i.e., 25% for first offense, 40% for second ... 100% after a few).
Of course, you need to prevent the \'workaround\' of a criminal handing off his/her goodies to a complicit guild-mate or chum who returns them after the penalty is enacted...
... so perhaps a Guild should have another penalty enacted upon it when a member breaks the law and is caught, e.g., Guild loses cash/loses stored goods/has a trade tariff imposed until a penalty is paid off, e.g., all goods cost x% more for ALL Guild members... this makes it worthwhile for the Guild to support/manage the good behaviour of its members.