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.
This is similar to house flipping, where a home is purchased and fixed up in order to sell for a quick profit. Unlike house flipping, however, there is essentially nothing to be done with the domain name in order to increase its value. Therefore, the key to a successful domain flip is being in the right place at the right time in order to acquire a valuable domain name before it is given a premium price.
That said, my recommended budget for beginners would be $500+. Using this budget you could buy a bunch of high-potential $10 domain names, expired/dropped domain names or a mix of both. It’s very important to invest only what you can afford to lose and treat this as a side hustle till you get the momentum going. As you gain experience, industry expertise and some sales under your belt, you can then consider slowly growing your business into a full-time gig.