There are two different tiers of Azure Security Center :
The Free tier provides visibility into the security state of your Azure resources, basic security policy, security recommendations, and integration with security products and services from partners.
The Standard tier adds advanced threat detection capabilities, including threat intelligence, behavioural analysis, anomaly detection, security incidents, and threat attribution reports.
The standard tier is charged based on the number of instances monitored monthly. The cost includes 500 MB of Disk space per day/instance.
For more information around the pricing of Standard Tier, please refer to this link