Have a chance to buy a MK9 like new. What would be a fare price?

Cabellas had one for under 500 with XS Big Dot Express sights installed as a used gun.

Well I did a stright across trade. The Kahr was new in the box, I traded my new glock 19.
I think I did ok

I'd say you did better than OK.  The value of a NIB MK9 is considerably higher than a NIB G19.

I wanted to bump this because i've got an opportunity to grab a BNIB mk9 Elite.  He's selling that it's worth about $800 bucks or so, which is more than my trade is worth ($600 or so).

Been looking to get a good concealed carry gun (that can be a pocket pistol too). Is this a good start for me?

The MK9 Elite is a nice gun for conceiled carry.  It is quite heavy for pocket carry.  If you really want a gun you can pocket carry as well as concealed carry I would take a look at the Kahr PM9 or CM9 if your trying to save a little money.  They are a half pound lighter than the MK9.  Also there are quite a few places on the net to get a MK9 Elite for $750 or less.  A couple places have it for a little over $700.  Even with a transfer fee it would be a lot less than $800.