There are a number of wrecking yards around here who get anywhere from $50.00 to 100.00 for what you describe, depending on how long they have had it laying around, what it came out of and if the guy working there is suffering a hang-over or not or if he's taken an instant dislike to you. (Don't laugh, I have first hand experience) A 3.0 open rear end would not be considered rare or desirable so it shouldn't be too expensive.
Some tips for wrecking yard scouting:
Don't: Drive your Shelby or anything nice. You will instantly be tagged as a money guy, the price goes up
Don't: Wear anything but jeans. Dirty jeans are preferable. Also, dig an old T-shirt out of the rag bag and put it on. Dirty hands and a little grease on your face completes the effect.
Don't: Attempt to pay for anything with anything but cash.