They don't cost that much if you are fortunate enough to get one directly from Rick, or win the lottery at Blade or other knife shows he attends. Rick sells them for less than that.
His production is far below the demand, so the second hand market is ludicrous. He also makes other models of knives, pens, kubatons, lanyard beads and other accessories - another reason for the low XM-18 production numbers.
About a year ago the second hand market for the XM-18 was $550. It now appears to be around $650, with some I've seen being as high as $750.
Some of Ricks approved distributors put markups on the knife also.
I do believe that's why Rick did the collaboration with Kershaw / Zero Tolerance - these new models very closely resemble the XM-18 (both flipper and non flipper). That should put more knives on the market for people to buy.
Rick makes really nice stuff, and he and Rob Orlando are really good people - I think patience is the word of the day when looking to enter the Hinderer club. It's taken some up to 4 years to find an XM-18 on the second hand market at a price they can live with.
Of course - he also conducts training courses at his shop and has get togethers on his property - and usually he has some knives available for purchase if you attend. Are you close to Wooster Ohio? If so check out his website - nothing like actually buying the knife from the man who actually made it.
Rob Orlando posted these prices recently on another forum (Blade Show 2011 prices):
3" XM-18 Production $385.00
3.5" XM-18 DLC BLACK Production $450.00
4" XM-24 Production $550.00
3.5" XM-18 Hand Ground Custom $550.00
4" XM-24 Hand Ground Custom $700.00
*CF Scales add $100
Those prices are VERY reasonable for a Hinderer.
Good thing you're not looking for an XM-24 - those are going for $1200-$1400 on the second hand market.