This is correct. I can't find the u-joints called out on the parts diagrams. But the below is also correct. U-joints are a common maintenance part that is readily available.I even looked up schematics from Nissan and they don't list selling just the U joint itself. Off the Nissan Parts website they have the U joint included in the journal kit propeller shaft. It wont let you just buy the u joint. Here is the link
You're more than welcome to argue, and to pay the ridiculous price. But if you call O'Reilly, Advance, Autozone, or Napa, they will all sell you a compatible u-joints from any of a dozen or so manufacturers for anywhere from $20-50 apiece, depending on which you choose and your location. Your mechanic is either not good at sourcing parts or he's full of crap.