How much does a small stain reduce the value of a $400 vintage 1960s Chanel dress?

I keep coming back to a store that is selling a 1960 first Chanel silk dress for $ 400, and I like it, but I’m hesitating to buy at that price, because there is a small stain (not at all unless you look very closely) near the bottom of the skirt. What is this dress really worth, given the spot, is $ 400 a fair price? Only serious answers please!