Again customers ... and maybe I was stupid

I also want to ask for advice. Made a site. We agreed on a price of 15,000. Ecoinstroy.ru At this stage, the design was changed from red to green because the customer did not match the color. (he said about green only after red) Below are the mockups that I did. The site has been installed twice. One under the orange design a second time under the red. Functionality is all done almost. He offered the customer to order the design on the side of another designer, only let him pay extra for the layout of that design. He said that he would pay 7000 on the condition that I would still stretch someone else's design on the site.

How much would have taken for the development of such a site? Thank.

Ps. They have a bikik that they need a lighthouse (“a lighthouse illuminating a sunken city (with animation)”)