I see 185s with some accessories for around the $200 mark often enough to say that is a bad deal on a used tank. My general rule is $1 per gallon and a bit more for any add ons like a stand or lid with lights.
i say no deal, i just got my 220 glass for 700 used which i even thought was a little high, but it came predrilled, with a stand, and had a pretty nice light fixture for corals plus the tank was pretty much brand new too, no scratches or anything.
The original poster is from Iowa. I dunno anywhere in the U.S where you can't get a brand new, reef ready, with canopy, stand, lights and hoods for under 3,000 dollars.
$600 is just pretty ridiculous for a tank, lights, maybe a couple filters and heaters and nothing else.