I'm generally skeptical of inaugural posts like this one, there is so much naivete in it that it's hard to take seriously. You're either being somewhat devilish in poking the bear, or you don't know what you don't know............likely somewhere in between.

That being said, the essence of your post in defining your objective is completely backward. It's infinitely more likely you'd spend $30k to have a product that would, hopefully, fetch $15k. People with years and years of experience, coupled with their "intelligence", find it difficult to sum up an all inclusive answer to you query. Each of the above responses attempts to give you some insight, and are generally accurate, but each represent just a singular slice of a MUCH larger pie. Generally speaking, anyone who makes any amount of money on a car sale in this segment of the hobby does it by applying some special talent they possess. And I dare say it is EXTREMELY rare to double their money.........and those are the best informed, most talented performers.

We have one member here who demonstrates the art of flipping hot rods. His special talent, from my observations, is that he is a highly accomplished painter with plenty of time on his hands, relatively speaking, as he's retired from a life of successful business ownership. And this observation I'm sharing is just a portion of what he does. He will often buy a nearly complete car for less than the cost of the parts to build it. He'll apply his painting talents making the car much more desirable, and then market it in the higher potential sales venues (rarely if ever at auction btw). I don't know what he makes on each deal, that's his business, but I'd bet you dollars to donuts it's not doubling his money in.

I have another buddy who has a talent for recognizing what's popular, and is one hell of a sales guy. He's willing to drive a couple thousand miles if necessary to take a car he's selling to a high profile event (just for example, Goodguys Del Mar) to find a "fat cat" buyer. In other words, he works for his money. I've seen him routinely turn a $5k profit (not considering travel expenses), on a $30-50k sale, sometimes a bit more, sometimes a bit less. And he's a guru at that sort of thing. One of his favorite phrases is "you make your money on the buy, not the sell...". In other words, you don't buy parts and labor at retail and expect to make money. Some other "intelligent" rodder has to eat the depreciation.

So there you go, another thin slice of pie................but there's oh so much more to know. Not trying to discourage participation in the hobby, and asking questions is a good way to learn, but don't think you'll outsmart the rest of the players in the game when you're just getting in it. Life rarely works that way.