i'd give about $500, it's used, it's an AMD based PC, that SSD is so-so, the ram is just bollocks (NEVER couple sticks of different sizes, models, speed);
SLI never really gives the same performance as a single GPU and .. is one of them watercooled?? does the system come with a WC loop?
$600 if the guy is your friend. sorry but it just ain't worth more. (unless there's a WC loop in there. then it all depends)