Jodo,
I paid about 700 bucks for mine, about 5 years ago, from Sarco, Inc. Here's a recent Shotgun News ad:
http://www.sarcoinc.com/2-10-10 7-13.pdf
The thing about Sarco is, you have to call the 800 number to ask what they have in stock...they don't advertise complete guns. I get a lot of my 'custom' parts from Sarco, too.
Be careful of gun shows: the prices are usually outrageous (a lot of guys try to get rich quick), and a lot of falsehoods fly from guys who don't know what they are selling....horror story: A buddy of mine bought a "Thompson" at a gun show for 400 bucks, which turned out to be a Volunteer Arms clone, which looks nothing like a real Thompson, once you compare the two...for a little more, he coulda got a Kahr Arms Thompson (like mine), who owns the Thompson trademark, and are the only company that makes new Thompsons today.
Here's a pic of one...you can see the differences in the receiver and trigger groups, and the bolt handle is on the wrong side...
http://farm4.static.flickr.com/3405/3211046445_18f9e79f4c.jpg
Keep in mind that the Feds require a 16" barrel on all firearms like this, unless you apply for the SBR (Short Barrel Rifle) permit stamp for 200 bucks, plus all the FBI paperwork (fingerprints, background checks, CLEO sign-offs, etc.), and having to buy the 10.5" barrel, to boot. I paid for all that because I wanted the WW2 "look"...it's a bit of a logistics hassle, but worth it (I think)....and the '28 Chicago Typewriter looks better with the 10.5" barrel, too.
Let me know if I can help with anything else, Jodo! :cool
Rob