It looks like the trigger point for the 15,000 points reward is a $100 spend per week for selected members. This is around a 19% return across the 4 weeks.
The 10,000 points offer for $50 week is a 25% return across the 4 weeks, plus whatever other points earned on docket deals and coupons during that time.
The lower targets do seem to still attract the highest percentage reward, but the higher target rewards are not as bad as they used to be. Someone on the $90 target might get annoyed however. Their return is about 14% which is less than the return for the person who has to spend $100 per week as mentioned above.