I too have seen several Martin 12's go in the $600 range on eBay.
Also, consider looking on eBay for a 20-year old solid top Yamaha (look
for an S after the model number)... lok for high numbers, 400's, 500's,
600's, they got for $200- $300 with case.
For example...
http://cgi.ebay.com/ws/eBayISAPI.dll...716778126&rd=1
I don't care for all the talking that partiocular seller is doing, but a few
emails asking about fret wear, etc would be in order.