Tell em what you paid for it ,when they are suitably impressed tell em how much you've spent on it ,stick on the time spent working on it plus 20 pc and that's how much your selling it for
Coincidentally, and as an addendum to yesterday's post, today (in a beach car park) I had "I used to have one just like that. Sold it. It wasn't any good, it was costing us money". I engaged in the conversation out of courtesy (after all, I was brought up proper ). Turns out it was got rid of 3 years ago...because, and I kid you not, "you wouldn't believe how they rust". I had no idea.
Can't be any worse than the bloke who the previous owners paid to redo our drive. I had to hire a scanning electron microscope to measure the thickness of the tarmac