How much would a css change for this site cost?

[URL=“http://www.constructioncdoucet.com/joindre.html”]http://www.constructioncdoucet.com
the site (not made by me) looks ok, and I might get an offer for changing the aesthetic aspect of it. No major layout change, just css tweaks mostly. How much would that cost the client? Sorry I have no idea…

Thanks =)