I think you'll find the 5.5" slabsides (KMK512GC) a little hard to come by. I believe they were distributor exclusives, either for Davidson's and/or Big Rock(?) Sports.
Other than the USST version (gaudy gold on bolt, fiber optic sights, etc.), I've only seen one 5.5" slabside in person- the one I bought from a forum member earlier this year!
I paid slightly over $400 for it shipped. It was used, in pretty good shape, with box/manual, but no fired cartridge (don't know if it's old enough to have come with one).
In the end, it's worth whatever you're willing to pay. I'd say $400-450 isn't out of line if it's in good shape and has got the box, manual, etc. with it.
If you're not
specifically looking for the 5.5" slabside, you should be able to do a little better (pricewise) on a more common model- such as the 5.5" bull (KMK512) or the 6-7/8" slabside (KMK678GC).
Happy shopping, and let us know how you come out!