Bobbi Brown and MAC eye makeup (mostly shadow)?

OK, I know they're both great brands, but I keep seeing people say that MAC is CHEAPER than Bobbi Brown, but when I checked both sites, I found that MAC is WAY more. So why do people say it's cheaper? With MAC, is there like twice as much shadow than Bobbi Brown or what? For example, I found a 20 shadow pallet from BB for like $70, but a 4 shadow one from MAC is like $50.