Price used depends on numerous factors - condition, mods?, case candy/accessories (like gig bag/case, tools, trem arm etc) included, year of manufacture etc. I doubt you could buy a new one - even with a discount for that price so its a saving. You can at least try it and inspect it, ask whether it includes all the case candy and all accessories and decide if the guitar is worth it to you. If it doesn't gel, then even at half the price its not worth buying (unless you can sell it for profit soon after) and may well be better off putting the money elsewhere. You may also be able to talk the salesman into offering a discount too of course.
Value is different for everyone and can depend on circumstances too. If you offered me 'book' price for any of my used PRS, I wouldn't sell because the are worth 'more' to me than that but if I was 'desperate' to sell a guitar, I could well accept book price or less. If I was buying a guitar, every one would have a 'value' based on what I am willing to pay, how much I want, need or even gel with a guitar. Its not always about 'book' price but what I am willing to spend. As an example, I would NEVER pay more for relic guitar because I do not like 'relics' (unless they have been naturally aged through time and usage) and to me, I do not value the procedure to artificially relic a guitar - A beat up guitar should cost a LOT less. If you buy a guitar that's been accidentally damaged in a shop, you often get a big discount. Again though, others are willing to pay extra for that because they 'value' that differently to me. For some, $650 may well be almost be pocket money - something they could just buy the guitar and not really care if its decent or just use it as a base for modding if it doesn't meet their needs, others may well think $650 is a lot of money and saved up for month's - the equivalent of some peoples Private Stock purchases. The value of money differs from person to person as well.
Point is, you have to decide if that guitar is at the right price for you, whether you are willing to pay that much, try and negotiate a discount to a better price for you or not worth pursuing. Its purely subjective and depends on what you think and whether the guitar is worth that much to you.
