Hello, Azure Security Center comes in two tiers, free and standard. Azure Security Center is a unified infrastructure security management system that strengthens the security posture of your data centers, and provides advanced threat protection across your hybrid workloads in the cloud. With the standard tier, you are paying for heightened security features as detailed here.
You can also view pricing estimates there. The pricing calculator shows detailed costs for you. You could switch to the free tier, but you will be losing the benefits of the standard tier.
Costs will fluctuate depending on your usage and daily rates based on availability. You can view your detailed costs by going to the "Pricing & settings" blade in Security Center, as detailed here. Your entire Azure cost analysis can be viewed by using the Cost Management tool in your Azure Portal.
I hope this helps! Please let me know if I can help you with anything else.