Contract termination is the customer's right. However, whether the agency's 'additional cost' claims are justified depends on the contract. Legally speaking:
1) Early termination by the customer — the agency cancels the service for the remaining period, and they can only charge a penalty if it's written in the contract. 2) Data and IP — all software, code, and designs paid for by the customer are the customer's property. 3) Hosting and domain — if opened in the customer's name, the customer owns it; if in the agency's name, a transfer is required.
Important: take a backup of all files (source code, database, images, design files). Make a written request to the agency: 'transfer all project files'. The site can be migrated to another hoster from the backup.