I want to know the breakup of cloud-watch service bill

Hi,

1. the metrics cost is prorated by hour and measured only at ingestion time (when you send data). When you swap, you stop sending data for the instance you're not using anymore so you stop paying for that old instance, and you start sending data for the new instance you've swapped to, so you start paying for the new instance. At the end of the month, the only difference is that at the time of the swap, you may have an overlap where during 1 hour both the old and new instances are measured, so you may have 1 hour of extra usage in addition to the 18 instances. 1 hour of usage is 1 / 744 (number of hours in the months, 744 is the example value for a 31 day month), so the prorated price for that extra hour of usage is 0.30 (metric price at tier 1 price) * 1 / 744 something in the order of magnitude of $0.0004 (so in that case it wouldn't likely be charged as it would round to zero, it's only if you have hundreds of swaps with 1 hour overlap every time that you'd see a difference to the bill).
2. Every metric follows the same retention rules. The metric is kept for 15 months, see https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_concepts.html#Metric - but after 15 days you don't see the metrics in the list anymore, so you need to remember its name/dimensions if you still want to query it
3. There are only 2 prices for metrics you get in CloudWatch: either they're free or they are charged at the price indicated on the pricing page (the price varies depending on your overall amount of chargeable metrics, and tier 1 price is $0.30 / metric / month of sending data to it). Metrics that are sent automatically by AWS services without any opt-in/opt-out option and are listed in the AWS namespace in the console are called basic monitoring metrics and are free. You only pay when you send custom metrics, or when you opt-in to additional and more granular metrics (usually called "detailed monitoring" or "enhanced monitoring" by the services that offer that option)
4. Retention is the same for all metrics, so the same logic applies to glue metrics than answer to point 2 above