EC2 Saving Plan issue - additional $400 in forecast

When you purchase a Savings Plan, you're making a commitment to use a certain amount of compute resources (in your case, $0.10 per hour) for the duration of the plan (1 year). This commitment is billed regardless of whether you fully utilize it.

The additional $400 you saw in your forecast likely represents the total commitment cost for your Savings Plan over the forecast period. For a $0.10/hour commitment over 1 year, the total commitment would be approximately: $0.10/hour × 24 hours × 365 days = $876 for the year

Depending on the forecast period you were looking at, you might have seen a portion of this total commitment. For example, if looking at a forecast for about 6 months, you'd see around $400-$450.

To correctly set up a Savings Plan that will actually save you money:

1. Analyze your consistent usage patterns to determine an appropriate commitment level. Don't commit to more than your baseline usage.

2. Ensure the instances you're running (t4g.medium in your case) are covered by the type of Savings Plan you purchase. EC2 Instance Savings Plans are specific to instance families in a region.

3. Remember that Savings Plans apply to eligible usage up to your commitment amount. Any usage beyond that will be charged at on-demand rates.

4. Check the utilization reports after purchasing to ensure your plan is being fully utilized. A utilization rate close to 100% indicates you're maximizing your savings.

If your instance usage is very low or inconsistent, a Savings Plan might not be the most cost-effective option. Consider reviewing your actual usage patterns over time before committing to a new plan.
Sources
Violations to EC2 Saving plans | AWS re:Post
Savings plan - not showing any savings and making sure its correct | AWS re:Post
Stacking of EC2 Saving Plans on the same Instance Familiy | AWS re:Post