The cheapest of these plans is $8.33 /month for 1 TB of backup space and support for 10 devices (and three sub accounts), if you pay for a whole year at once. You can add more for the same price and storage, like another 1 TB for another $10 /month. The maximum storage plan you can pay for is 15 TB, but if you want more you can contact their support team.
If you want to download as videos itself, you can use other free tools like Free Youtube Download to download all videos with a single click. Please be careful while installing this tool as it optionally installs a browser toolbar and modifies search preferences. So please make sure you read and uncheck the optional checkboxes while installing to avoid surprises in your browser.