I was recently working with a customer who was seeing double the quantity of a configured resource under Costing > Prices tab on an instance vs the Instance Summary tab.
For example, the total storage on the instance Summary tab was showing ‘40GB’ (which had been configured in the instance provisioning wizard when the instance was originally provisioned) whilst the Costing > Prices tab was showing ~80GB.
The explanation for this is that they had Price Sets attached on the layout-level. This is expected behaviour since a fixed price is added to the layout and the final computation of quantities + prices takes into account this fixed price alongside the pre-existing service plan used.
We have a helpful video that can be used as a starting point if this functionality is required: