That's probably more or less true. But the factory IIC+ DRG(X) models (Simul-class, reverb, GEG, (export tranny)) are the ones that command big bucks. Missing one of those options, the price plummets. Then add that it's a conversion to + spec, and the bottom really tends to drop out.
But, collectors are the ones driving the prices. Arguably, this amp sounds darn near as good as any factory DRG, so it's good amp for a player to go after, but my point is that a converted SRG IIC+ like this one can be found somewhat commonly in this price range. Not a terrible price, but close to if not somewhat above market.
I actually "bought" a factory SRGX IIC+ for less than this a few weeks ago on eBay, but I let the seller out of it because I decided to try to just be content with what I have and he got a higher offer after the auction closed. Just saying that better deals can be found.