There's a few things to consider. Clones are usually always built to the desires of the builder and not the buying public. Things like color, interior, powertrain, etc, are done to meet the needs of the builder, and only the buyer. I've seen lots of guys dropping $$$$$ into cars to make them their dream car, then get upset when a buyer without that same dream only wants to pay $$. That's the risk when you modify your vehicle.
I also never take into account how much it costs to make a car myself. I know what it costs, and to it is irrelevant as all I'm concerned is what I can buy the car for. There is no such thing as "fair value" when I am buying. There is only absolute value, which is what the seller can be convinced to sell for. Am I screwing the seller? You betcha, but that's the nature of buying and selling. I think if anyone expects to get more than 50% back on their investment in building a clone they are nuts. If you spend $30k, you should expect to get $15k, and if you get more you may consider yourself blessed.
Lastly, we all get lots of offers for our cars, but what usually happens is when you say "Okay it's a deal" the offer maker either disappears or suddenly has health issues, wife issues, job issues, dog issues, or some other issues that makes that offer null and void. Someone says they were offered $40k, I would ask "Did you see the money?" When they say "No" I say "Then the guy wasn't making a serious offer."