Britfish, I can't say what dummy '28 Thompsons are going for in your neck of the woods, But I CAN tell you what I have in my M1A1
Thopmson M1A1 kit ................................................$350
Richardson receiver w/engraved logo etc. ...............530
New buttstock,forend,grip........................................ 62.50
Bluing on kit parts..................................................... 50
Total $992.50
As you can see it is pretty darn expensive to create a "new" dummy gun.