Jeez.. the last time I looked for an m6 they were going for ~$350, and that was about a year ago.. now they're easily $4-500.  I'd be happy with one of the Marlin .22/.410 O/U though.  Not as cool, but still a nice gun. 
Try Cabelas gun library. Online you will only see firearms priced $500 and up.  If you call, they can research any $ amount.  This is how I got mine from KC to here in DFW.  $250 for the gun and $50 for shipping.  Be patient cause it ships on their store truck, and that fellow gets paid by the hour vs. mile I think.
The real kick in the tail happened in late 1990.
I was offered 5 of the USAF issue M6 survival rifles (14" BBLS) at the cost of $100 ea., buy a "Fine Upstanding Young Man Yes Indeed" (READ: Drug soaked bum).
His only mistake was not realizing that I was an off duty MP less than 1/2 a mile from the main gate of the post. . . . . 
          Toughest honesty call/gut check I ever faced off duty!
Still no relationship with aforementioned retailer except wish lists.