The monthly budget cap should be allocated to existing storage first. This covers the existing data for the next month. If there is any free limit left, it could be used for new data writes + 1 month of storage, and/or running services. Once the limit is reached, then writes are blocked and services stopped.
The only situation where you would need to delete data is if you want to set a new monthly budget that is lower than your existing monthly storage-only bill - but the UI could just disallow this.
The monthly budget cap should be allocated to existing storage first. This covers the existing data for the next month. If there is any free limit left, it could be used for new data writes + 1 month of storage, and/or running services. Once the limit is reached, then writes are blocked and services stopped.
The only situation where you would need to delete data is if you want to set a new monthly budget that is lower than your existing monthly storage-only bill - but the UI could just disallow this.