A lot of North African 'saffron' is actually safflower, similar in colour but with a more 'petaly' shape as opposed to the strands of true saffron. To add to what everyone else has said this might give you an idea of quite why it is so expensive, 'To glean an amount of dry saffron weighing 1 lb (450 g) is to harvest 50,000–75,000 flowers, the equivalent of an association football pitch's area of cultivation; '
PS. I live in Saffron Walden which used to be England's major saffron cultivator but it died out in the Middle Ages.