Can figure average prices comparing swapping rear ends per simply narrowing...
Average price of narrowing rear end $300+ (That is if can have axles resplined)
Average price of a new/used rear axle $400+
Reyoking a U-joint and turning/machining driveshaft to fit new rear end $40-$80
Also depends if you get lucky and shop around and find the best deals, or you stumble on someone that is 'getting rid of a rear end' that you just so happened are wanting and also how much work/knowledge you are going to put in also? Like I mentioned go to some salvage yards with a tape measure, do some research.