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.
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.