NOTE: OMS licensing is changed, please look Security & Compliance pricing .
OMS licensing is very flexible but sometime you can feel lost guessing how much will cost adding a new server or a new solutions.
As always, having a testing environment can help.
A good way to understand the amount of data you are about to put into an OMS Workspace and how much is going to cost is to create a secondary Workspace on the Free tier. Here you can test new machine/solution/data collection rule you would like to estimate in size with real-world data.
This Workspace works as a sandbox where you will discover how much data is going to be added with real data from target environment without incurring in any charge. All you have to do is to add (or multi-home) yours target server to the testing Workspace and then add Solution/Data collection on it.
You have to remember that Free tier is limited to 500MB/day (UPDATE: not for Security & Compliance).
Even if this limit that can be easily reached, especially with Security and Audit solution on Domain Controllers, you can leverage the obtained data to understand and better estimate the amount of data and, based on your license tier, the impact on your costs.
The Usage page is a very powerful way to understand where data is coming from; there are views that quickly highlight Data Volume over time, by Solution, by Computer and not associated with computers.
Even more, like almost everything in Operations Management Suite, you can use Log Search capabilities to dig into your data to collect all the information you need.
Last but not least, you can leverage what you learned on the testing environment to correctly set Alerts when the amount of data is over a certain threshold to being informed or, through Automation, trigger action to analyze a strange behavior in data generation on server/solution/data collection rule.
As already said, having a testing environment should be a golden rule to be always used in your way of working. The Cloud make it easier, cheapest and faster to build, use without worries about maintenance.