I Can't speak for what other people are selling or buying them for but both of mine were bought at cost with a little patience and good timing !!!
Lucky you!

After looking for nearly 2 months, the best I've seen is $500 shipped CONUS, and since I'm in Canada that becomes $525 minimum, plus brokerage, plus taxes. Bottom line, $600+. Still looking.
Seriously though, for me this is the deciding factor between the SNG and XM18: availability. Given a choice of $450 for the XM-18 or $350 for SNG, I would go for XM-18, no question in my mind. But given a choice of $350 for SNG or $600+ for XM-18, I have to say the SNG is the more reasonable choice. Either knife is more than tough enough for the average user to warrant the "indestructible" label, either one can cut, and while the fit and finish on XM18 is better, is it "double the price" better, especially for someone who intends to use the knife?
Anyway, that's my opinion on it. I only got to handle the XM18 once, and I've been looking for one on and off ever since, but I just can't pay over $500 for one (including shipping, etc.) At $600+ you're in a custom knife territory, you could get a custom for that, with an even superior fit and finish/materials.
EDIT:
And yes, there's many reasons why the XMs are hard to find and why the secondary market prices are the way they are. And I totally understand why people are upset by this. If I was standing in line for 2 years and someone skips ahead by being a firefighter/LEO/military (which is fine and fair and speaks highly of Rick's character), and then immediately turns around and sells it for a neat 40-60% profit, it would tick me off too. That's not the point. The point is we all have to deal with realities of life.
The reality right now is that SNGs are plentiful and readily available for $350-400. XM-18s are rarer and take time to find even at $600, depending on where you are. And while it is indeed impossible to measure a knife's worth relative to another knife, there are limits. You will not find a knife with 420J2 blade and FRN handles selling for $600. It just doesn't happen. In my opinion, $600 for the XM-18 crosses that imaginary line, at least it does for me. I find it hard to justify $600+ for XM-18 when I can get a custom for that much, with even superior materials and at least comparable workmanship. Beyond a certain point, it's just not worth it.
And while the thread asks on the opinion between the SNG and XM-18, I would like to suggest people consider the new Ti Spyderco Military. Look at it. Full Ti handles, RIL mechanism, S30V blade, full flat grind. What's not to like? Is it a tank like the XM-18? No. But will it fall apart if you sneeze at it? Not at all, I've beat the heck out of my old 440V Military with linerless G-10 handle and I still have it today, for my needs it's more than tough enough. Is the fit and finish on it as good? Debatable, but let's say no. Will it matter after you put the knife through the meatgrinder of daily carry for a year or two? Again, no. What does it cost? $200. That's HALF of the SNG, or 1/3 of a second-market rate of the XM-18.