price should be per axle whenever you see them unless they list a kit. installation kit is good because bearings and shims may be worn and not meet tolerances. also, be sure to ask if they are course or fine spline pinion. some companies are only offering fine and you have to buy a companion flange also.
The 4.56 kit for '78 at $179 looks like a smokin deal, but probably doesn't come with the flange. The extra money for the flange for the other years is not a good deal because you can use a mini truck flange and just drill new holes for your drive shafts. You need an installation kit if you want new bearings. It is hard to get the inner pinion bearing off intact without a press. You will need a crush sleeve to set the pinion preload and an extra one if you screw it up the first time. Most aftermarket R&Ps are designed to use crush sleeves.
I bought a R&P from Randy's, they sent me fine spline, I did get an adapter flange, but the pinion gear was a different length and nothing fit right. Hence the 3rd member purchase from David. So I'm not sure that a companion flange is all you need to make a fine spline R&P work in a previously coarse splined rearend. ???