In your area, mixte's won't come cheap unless you are both lucky AND quick. For $150 or less, plan on getting one that needs a complete overhaul.
I just sold a low-level Centurion DLX mixte a month or so ago for $220. Of course, it was completely overhauled, cleaned and ready to ride. The woman that bought it came all the way from Santa Rosa to get it. They are popular, and clean ones go quickly.
That being said - the $520 one is a joke, and I've seen it listed before. It won't sell at that price. But $200-$300 for a CLEAN ready-to-ride one is the ballpark you're playing in......
You won't do much better than this one (not mine), and I'll bet you can get it for $250 - were I looking for a mixte I'd cough up the dough and be done with it. You can buy a wreck for $150 and spend time and $100 to overhaul it, or save a lot of trouble and buy a clean one:
http://sfbay.craigslist.org/sfc/bik/1230200674.html
I'll keep my eyes open for you - any size/make/color preference?