Used Sony RX100 (Very Good condition) for $520
The Sony RX100 has been stable at $650 at authorized and reputable dealers. If you are an expert photographer and expert shopper, you may want to trade off the possibility of used from Amazon Warehouse Deals for $520 in Very Good condition. Look for Amazon Warehouse Deals as the seller - they offer a 30 day return policy. There is no warranty but you can optionally purchase one from Square Trade at the aforelinked pages. Again, let me note, do not buy this just because it is cheaper - make sure you understand and you are comfortable with the trade offs of being used vs new!
For a refresher on the camera, check the list of completed Sony RX100 reviews.