Billing Rules can have an effect on the values that are shown within Rightsizing reports. Below are two columns to consider:
Savings Recommendations: It currently uses the on-demand price for the recommended instance, and the billing rules cost for the current instance, to compute savings. As far as billing rules are concerned, it’s half-included in the savings calculation. The recommendation’s cost is the on-demand cost so savings is calculated by doing “existing cost - recommendation cost”.
Projected Cost: The projected cost is strictly the on-demand cost, no billing rules included.
The result is that the Rightsizing report will probably under-estimate savings, and over-estimate projected cost, if they have billing rules enabled.