Stolen from the TGR forum, but its a good start...
50% of current selling price (not MSRP)
-10% per year old
-10% per times drilled, per core shot, per delam...etc
-10% per marker binding attatched
+10-30% for Rare/High-demand item
Id say the 50% off selling price is a little much, but Id take off about 15-20% because they are used and drilled. Id say $500-$550 is fair if they are selling at $700