A rule of thumb that I follow is when I purchase a domain name I have to believe that I can flip it for a 100% profit. So if I spend $500 on a domain I need to feel like I can sell it for no less than $1,000. This way, even if I’m wrong in my estimation I still have some room to still turn a profit. And if worse case comes to worse case I’ve sold domains before for a $0 profit (it happens even to the best of us).
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.