I've talked with a couple yards about buying axles or parts of them, most around here want $150-250 per axle, but some of them I've managed to beat down on price when I've needed it. When I changed gear ratios in my F-150, they gave me a price of $200 per axle. I beat them down a bit then wandered out in the yard to look for some things. Came back in the office later and sat down to dicker a bit more on things. Ended up taking most of the day (had some beer with the guys running the yard), but I got the axles for what I wanted. Some yards are non-negotiable though, so I tend to avoid them.
But making friends at a yard is usually a good idea, off at college I managed to make friends with the owner of the one yard - it was one of those that you don't get to pull your own parts, but since I made good friends with the owner, I got to pull my own parts and even got to use their shop with the lifts from time to time. I even got to strip out a wrecked BII, $150 and I got to take anything I wanted except the rims, tires and computer.... so I took everything except the frame, shell and seats (which were junk) and I do mean I took everything - entire drivetrain, glass, doors, hatch, steering box, etc. Then I used their shop to put the engine in my choptop, lol.