Hi Deval,
If you wish to use the AdWords API, you will first be required to sign up for a MCC account so that you can then generate your developerToken from that MCC account. You can carefully follow this
sign up guide on how to achieve this and afterwards request for
basic access through this
application form so you can make calls against your production accounts (once basic access has been approved) using the AdWords API. Also, make sure that you generate your OAuth2 credentials by referring to this
guide.
Moving forward to your concern regarding access, as long as the users are not using the OAuth2 credentials which you created while logged in to an email address which has access to your MCC, then they would not be able to access your MCC account and all other client accounts under your MCC. They would need to use OAuth2 credentials which has access only to their (client) accounts so that their access will be exclusive only to the accounts which the email address has access.
Regarding costs, when your production account's campaigns, adgroups, ads, and keywords, etc. start serving or when they start generating statistics, that would be the time where you will be billed. For further assistance regarding how your account will be billed, I would recommend that you get in touch with the product experts via the
Google Ads Community Forum as they are better equipped to assist you regarding this matter.
I hope this has been helpful.
Thanks and regards,
Peter
AdWords API Team