Each week, everyone is taxed 50% of whatever they've gained (other than purchased Karma) over the previous week less whatever they've spent in the store.
So, Taxation = (50% of income) - (amount spent in store), no negative taxation, of course.
25% of the total taxation goes into a lottery with multiple winners taken from those who spent more than 50% of their earnings over that week in the store.
I'm sure we can find lots of variations around this kind of theme.