CloudWatch Alarm on Specific Group of Instance

I am very new to the aws Cloudwatch and my usecase is like I have 50 instance running in specific region, Now I have to add the billing alarm on only 10 instance from the group of 50 instances.

I don't know weather it is possible or not because the billing can only be applied on the all instance not on some specific instance.

Is there any way to do it by editing the metric source to filter the required instance ?

Thanks !