Honestly, the answer is simple. They ask.
Not to be annoying or anything, but that's it. There is way to much competition in the web design market when you are competing by price. I don't care how cheap you can do it for, someone can always go cheaper.
You don't compete on price, you compete on perceived value. When I first started out, I was selling wordpress sites for about $300-500, and I thought it was amazing. I was making over $100/hr, which was fantastic. Then I met with a consultant, and she was doing the exact same thing, but selling them for 3-10k. She said don't do a website for under $1,500. She was actually outsourcing the web work to India for $250, so she was getting the site built for a ton less than the person paid, and just collecting all the profit on top.
So yeah, just ask for the price. Don't scam, but if you honestly see your site being worth that much, then you will find someone to pay that much.
Good luck.