My buddy is trying to sell his Thompson Contender 338 JDJ, it comes with a Burris 3x-9x scope and he also has the shoulder holster. What is the most he will get for it?

His gun looks just like this and has a scope. I guess I should have checked here first. Anyone interested?
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=418210426

Is the frame a Contender, G2 or Encore.

If the barrel is custom made from JD Jones, Bulberry or Bellm,,,,,

I would guess for the Contender or G2 frame with what you listed:
$175-$200 for used frame
$200-$250 for a good barrel with minimal blemishes
$200 for the scope, even though new the 3X9 can run in the high 300's to low 400's

Low end $575, high end $650.

With the Encore action, I would add $100 to that.

I also have nine of these single shot contenders that I will need to let go of half before long.