Gregg Miner often has multiple harp guitars for sale, you can check here and look at current and past stock and prices,
http://www.harpguitarmusic.com/instruments.htm#sale I bought my S12 from Gregg, and he's put together a huge amount of great info on harp guitars on his sites.
I was asking more or less the same question before I got my S12, and the answer seemed to 2-3 grand, with Tonedevil being the main purveyor of really good quality and relatively inexpensive wood harp guitars. Emerald is in about the same range with carbon fiber harp guitars. My S18 was a custom job, but only a bit more, 3.5K. You can of course pay far more. You can pay much less, but then you've probably bought a cheap Asian knock-off that you won't be happy with unless it's just an interesting conversation piece to put on the wall.