Well, it's worth what you'll pay for it, I guess, but a clean bare housing is worth maybe half that to me. For $150, it should at least have axle shafts and it better be really, really nice. I get 'em complete from my favorite wrecking yard for $75-$125, pig and all... regardless of casting number, diff type, or gear ratio I've never paid more than that.
DD2, there's no such thing as a factory 4.5" bolt-circle A-body 8.75", so if you hope to find one it's going to be one someone converted (hopefully not by redrilling the axles). With a 3.55 and a Sure Grip, you're near the four-digit price range. When the economy was stronger, I'd seen them go as high as $1800. Your best bet is to find a '65-'67 B-body axle. It's just a little bit wider but a whole bunch cheaper, and A-body guys have been using them for years. A 3.55SG pig is still a pricey proposition, though... that's the most sought-after ratio, based on when I was selling 'em. I've probably had twenty-plus 3.55 gearsets, and they always brought premium bucks particularly in an assembled pig. 3.91s and 4.10s were popular as well, but never seemed to bring the money of the mid-3 gear.
I never did keep a set of 3.55s for myself. :doh: I did, however, keep my NOS 3.36 gearset... ideally suited to an A833OD trans.
Best bet? Hit the boneyards, and look in pre-'75 trucks, particularly Sno-Commanders.