At the beginning of the month, it is expected there will be a large chunk of Unused RI as this is what needs to get used over the course of the month.
- You may notice a spike in Unused RIs at the beginning of each month.
- However, day-by-day as the RIs are being consumed you will notice this cost decrease as your Compute charges increase.
For more details here are 2 useful links:
What are 'EC2 - Unused RI' costs?
How does the Amortization cost report work?