Ahh, well there in lies the trick of it.
You see the Bren Ten's are relatively rare- they only made circa 1,500 of them. And some of those were in a different size, or make to the Standard 10mm Auto Version that was used by Sonny Crockett (yes yes he used a blank firer in .45 for the tv show but they are even rarer with only 250 dedicated models made)
So you have to do your home work - figure out what is a fair proce for what you want and go out and hunt for it.
You are better to assess a deal from a distance then start asking for one-people may add $$$ to the price if they think you are desperate to buy.
http://www.gunsamerica.com/
or
http://www.gunbroker.com/
Advertise them all the time. The Jeff Cooper Commerative currently for sale has a number of things going for it - the seller is a trustable source. And it's an original.
UNLIKE the recent Jeff Cooper Commerative that was advertised for $20,000 recently and was a fake.
It's a jungle out there - so good luck.
BT