From what I can tell it is based on how much the car is renting for on the day you are searching on.

I have never seen a rental going for 400 points a day. The lowest I have seen was 500.

For ex, MCO atm is giving a economy for 1000 points a day from Oct 2-4.

I have been Platinum with enterprise with 100+ rentals a year for 2 years and maybe got 5 or 6 redemptions. From what I can tell the points are hard to come by even with the double point promo.

The locations I go to are not pushy about upgrades, upsells or insurance and cs is very good overall.