My company have decided to move from SharePoint Online to SharePoint On-Premise because the manager see it's costly.
Is it a correct decision? And whatever the answer, how can we move from Online to On-Premise Foundation
It's depend upon the requirement, but as you mention, you are moving to SharePoint foundation which is clearly due to cost.
For migration you can move the content only and for that
Either Manually Move The Document /files to on prem(using open with explorer)
Or Use 3rd Party Tools for migration, I would use sharegate( they have trial so you can try it)