According to that, there is an extra "9" they didn't show, so $3.99 rather than $3.90.
So then if you deleted your 1TB each month, and uploaded new data, you'd be paying 3 times more due to the 90 day minimum charge, so about $12/month in the worst case.
That is to be expected for bottom-barrel prices, to make up for the cost of maintenance (GC / compaction / rebalancing / etc of data). They might just wait 90 days before doing the first compaction, or whatever their system does to delete data.
