How to copy a Google Sites Website.
If you created your site under site.google.com/site/ and you want to increase site storage or you want to create a backup of your site, here are the steps.
Copy your site which you created under sites.google.com/site/ (using a non Google Apps account) to Google Apps for Work.
Copy your site created under site.google.com/site/ to sites.google.com/site/ as a backup copy.
Make a backup of your site under Google Apps for Work to under Google Apps for Work.
Make a copy of your site
Note that site which is created under site.google.com/site/ offers only 100MB of space storage. Read more on Google Sites storage limitation, file size and create site limitations.