A post like this (or for Premium Room Rewards) comes up every month or so. I will repeat what I usually post.

The points cost of standard rooms is based on the hotel's reward category. The points cost of premium rooms, upgrades, etc. is based on the cash price (or price differential) and assigning a fixed value per point (which I believe is somewhere around a third of a cent). If the cash price is high, the points cost will be high, and vice-versa (to the extent that sometimes premium room rewards come out cheaper).