You will have to match whatever the going prices are in the venue that you sell. If using Ebay, do a search with "completed listings only" displayed, which will tell you what they are selling for. If using Craigslist, you'll have to match whatever asking prices are there.
If you are not in a hurry, you could always try for $700 first for a couple of weeks and then drop it down later if it doesn't sell. Most people seem to be in a hurry though.
I've sold two Minis in the past and I've found that if you still have the original box, discs, manual, Apple stickers, etc., you can easily get $30-$40 more than what others are asking. (So when people throw away their boxes they are throwing away $40.)