Are they expensive because of some inherent property, or because they're only used for prototyping and are thus done in small runs? Having spent several hundred dollars on a little plastic BGA socket that could be made for pennies, I'm inclined to say that it's the insanely small production runs right now.
BGA balls are made of solder, and they'll cold-flow under pressure. So any socket meant for durable use in production equipment (as opposed to prototype or test purposes) is going to require a lot of small, delicate, and expensive spring contacts that provide a secure connection with very little force.
Also, sockets in general are perennial problem areas for signal integrity and EMC.
It is probably best to think of the motherboard as a monolithic hunk of material, going forward. Too many reasons for the trend to go in that direction.
