Forget the headers, run manifolds. Do the wiring yourself, farm the PCM out for $75, run a Corvette fuel filter/regulator, frame rail mounted pump. Lots of fab work, but it's doable. If you're not completely comfortable with anything I've said, you won't do it for less than 2k let alone 1k
I agree with you to an extent. It depends on where you are as to how unwanted that small block Vortec engine will be. When I worked at the junkyard (I'm in KY) I remember how rarely we would get a Vortec in and when we did they were usually priced pretty high. The last reasonably low mileage Vortec engine in stock was priced somewhere around $900. I'm pretty sure that I could have gotten a 4.8 or a 5.3 for less than $600. Of course a pull your own place would probably be a lot cheaper.
As others have already mentioned, the engine itself can be acquired for a reasonably cheap price but it's all of the other stuff you need that makes the cost go up quickly.