What the heck is up with 8-3/4" rearends?

Like anything else. Supply and demand. If you have a A body rear and sell it to someone for say $500, The next guy hears about that and asks $500 for his or maybe more and may get it. When the prices get so high that no one buys them any more, the prices will come down. Even faster if someone comes up with a cheaper alternative. (like the 8.8 Ford) Don't pay the big price, be patient and wait for a deal, or find a different way.