hi there what type of brakes are we talking about,sounds like conversion bits too me (honda,camira,datsun,etc) as classic mini new components and second hand ones(caliper especially) dont realistically become availiable for the prices you have quoted
main reason for costs of genuine bits is weight and freight and the dreaded uk pound as they have to be imported from overseas
also a bit more than backing plate as you need different,hubs,drive flanges,drive flange collars and screws,discs,pads,cv joints,tapered wheel bearings,calipers,(caliper kits/pistons if need recoing),it unfortunately all adds up pretty fast
hope this gives you an insight into perceived high cost of brakes,probably the most important safety aspect of the vehicle to save your life
ps: will probably also depend on what you call expensive as disk brake kits will vary from retailer to retailer depending on the type
cheers

Do you have link to the other post you are referring to? That might make it easiier for everybody to understand the context of your comment

whoops pressed wrong button
should have been reply to high cost of disk brakes