Many organizations implement SAM as a defensive measure to ensure they do not receive a nasty surprise at the time of a vendor audit. Although this makes good business sense, it should not be the only reason why you do it, as the cost savings alone can provide a very compelling ROI argument.
Reduce overspend on hardware and software
The most obvious cost saving will come through reducing overspend on both software and hardware. Probably the surprise here is hardware, but the fact is that the disciplines of a strong SAM programme will help identify where hardware is being over deployed and show where consolidation, such as for servers, can be effectively implemented to reduce costs.
Similarly, monitoring software usage will enable you to identify where software is not being used and so re-harvest the licenses for use elsewhere. This will help ensure that you only ever buy and deploy what the organization needs rather than what employees think they require.