Expired domains are domains that have been registered by individuals, businesses, or organizations, but aren’t renewed after the contract ends, or are deliberately terminated. That means that they available for re-registration. There are many reasons why someone would choose to abandon a domain; one reason could be that the web project failed, or that a domain portfolio went into liquidization. Domains can also end up becoming free due to the owner receiving warnings for possible trademark infringement. Re-registering can also result in legal consequences.
In theory, you could technically acquire a domain for $10,000 and be able to sell it for $100,000. You could also acquire a domain for $10 and be able to sell it for $100,000, $10,000… or $100, you get the picture. Because of this, it’s hard to recommend a perfect starting budget because your strategy, experience, and the pace at which you learn will hugely affect your buying and selling decisions.