So when using Spot block , if price of the spot instance varies , will it be charged with changed price or the initial price we booked it with ?
To answer your question, what you would be charged the price that you booked the Spot Block with. However, that won’t matter for much longer since Spot Blocks are being phased out: "Update July 2021 – Spot Instances with a defined duration (also known as Spot blocks) are no longer available to new customers as of July 1, 2021. For customers that have previously used the feature, we will continue to support Spot Instances with a defined duration until December 31, 2022. If your workload is interruption tolerant, we recommend that you use Spot Instances without setting a defined duration. If your workload is not interruption tolerant we recommend that you use On-Demand instances for the required duration of your workload."