Will we incur additional costs in maintaining the archival?

Yes, since the archive is independent of your Sendgrid service, you will need to pay the archival vendor, such as Vaultastic. 

Vaultastic is designed to optimize the costs of your ever-growing storage needs. It delivers up to 60% cost savings through methods like data deduplication, compression, storage tiering, and other efficiency measures, ensuring that you can securely archive your data while keeping costs manageable.