The market-driven principles of the domain trade mean that a domain is only worth as much as the buyer is willing to pay. It is for this reason that criteria such as market potential and usability play such central roles in determining prices. Values can change at the drop of a hat. The price of a domain that was once of little interest to anyone in years past can skyrocket once, for example, a newly founded company takes interest in that same name.
I'm not against making money in any way, but $400 doesn't really seem worth the effort and coffee expense invested (I like the good stuff). This is a very basic example used to put the costs in perspective. Most SEO providers charge more than $50 per hour, and you get what you pay for. The above example of a final labor estimate is probably much higher, or if the domain is already ranking high and the owner wants to sell, so is the initial purchase price. Since this is often repeated many times over for multiple domains, it could get time consuming, and expensive.
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.