Perhaps you will find it riding on a unicorn too? Unless you are talking a 4l60E now, even then $1000.00 will be tough for a rebuilt. Besides if you are talking A4 a standard rebuild won't stand a chance against your engine. IMO you'd be better off just getting a built one before you attempt the swap. In the long run it will be cheaper too. This is where I got my 4l60E, they are fantastic to deal with and they don't do core swaps, everything you get is brand new so you can still sell off the old stuff.
http://phoenixtrans.com/html/gmtrans.html