When trying to flip a car it's easy to burn through money like you said. Even if you know the car and the market really good it's super easy to go over budget and loose money. The reason for this is: when you buy a car you'll know the obvious problems if you know the car type well. Some problems are easy to see and can be fixed by someone at home.
However you don't know everything wrong with the car until you've driven it 100 miles. You just don't.
The tons of little things that are wrong that you didn't know about when purchasing are what add up. No matter how good you are there's always going to be something wrong that you didn't know until it's too late. Know this and chose wisely based on this advice.
Hope this helps.