Travis,
I suspect the $50 axle assembly is probably a remanufactured unit. They by the parts wholesale and rebuild them very cheaply...and often in other countries where labor is cheap. The local price is probably after exchanging your old axle.
Most axle joint failures are at the outer CV Joint, so the cheap rebulder may simply put fresh grease and a new boot on the inside CV Joint, and replace the bearings, grease and put a new boot on the outer CV Joint. It probably only cost them and average of about $20 to rebuild an axle.
There was a local shop (closed now) that offered to replace the front axles on my old 1990 Honda Accord for $110 per side (including labor). I did not do that since I only needed to replace a torn boot....I replace the boot myself, but afterwards, I thought it would have been worth the $110 to have them just replace the axle....regreasing and replacing the boots is a messy job and why they just wanted to replace the axle rather than the $10 boot and $100 for labor?
...Riich