It's in high demand so the price system rations it to those who really want it.
Anything labeled "prime" will be more expensive. It has higher fat content through the marbling of the meat. There are only two rib roasts like that per animal and it is a desired cut of meat which also drives up the cost.
Because it is prime
When you figure that 8 lbs of meat will feed 10 to 12 people and still leave leftovers it's not that bad.
One thing you can do in the future to cut your costs is to make some really rocking side dishes. If you do some really awesome oven roasted potatoes, homemade apple sauce and a baked ziti or lasagna along with the prime rib then people won't mind if you serve a little smaller portion of meat.
I worked in restaurants and catering when I was younger so I know all the tricks. Next time buy a smaller prime rib and make up a baked ziti. You could also add a mixed green salad, green bean casserole and garlic bread.
because there is so little of that cut on a cow