Just my opinion, but assuming Chantilly vs. a post warning (read directions before firing blah, blah, blah), I would say that if both were in identical great shape, you should see a difference of about $300 not counting the laser sight.
Here of course is the difference. You will often see a brand new or almost brand new M8 being advertised on one of the forums, which will say "low mileage, almost new, etc. And because the owner paid $1600 for it new, he thinks he can get that on the used market. The NEW retail prices went through the roof when HK stopped manufacturing P7M8's, but the used market hasn't caught up with this. Your find is very unusual. Generally, an internet "good deal" on an M8 is about $1100.
Great find on the M8. Now, buy that Chantilly, and it can be your safe queen.