There are four (4) options for calculating Jira Cloud prices:
Use the Atlassian Cloud Pricing Calculator. First choose between monthly/annual, enter the # of seats your Jira is licensed for, select Apps, select Git Integration for Jira (Top Selling) from the list of dropdowns.
See pricing details on the Atlassian Marketplace: Git Integration for Jira
Install the Git Integration for Jira then go to
https://replace-with-your-Jira-Org-Name.atlassian.net/admin/billing/estimate. The estimated monthly or annual price will be quoted.
Contact Atlassian to create a quote. You can expect an answer within a business day.