Supplier price management
The first thing to do, of course, is to consolidate the billing of the Cloud consumption of the different Business Units into a single account in order to better track the expenses and manage them centrally.
This also allows you to benefit from price reductions thanks to volume, as is the case for example for AWS Enterprise support costs, which go from 10% of the spend to 3% of the spend depending on whether you consume less than 150K per year or more than 1M.
Next, consolidated consumption must be planned so that annual spending commitments can be made that will result in rebates under supplier "Enterprise" programs.
At AWS, for example, this is the Enterprise Discount Programme, at Microsoft it is the Enterprise Agreement.
And of course you need to reserve capacity for both compute and instances using services such as AWS Savings Plan or Compute Pre-Purchase at Azure. Huge savings can be made by combining the needs of several business units in terms of similar instances if they are booked for one year and, of course, even more for 3 years.
See also "2O3 - Reservation management".