Why is Noble Hierarch so expensive?
Look at how much each has been printed. Noble Hierarch was printed in Conflux and Modern Masters. Birds of Paradise was printed in Alpha/Beta/Unlimited/Revised, 4th, 5th, 6th, 7th, 8th, and 10th edition, Magic 2010, 2011, 2012, and Ravnica. There are just way, way more copies out there.

And by the way, Exalted can be quite relevant. The most obvious example is Infect (quite popular and can win tournaments), which is likely to play only Noble Hierarch and not Birds of Paradise. It doesn't need the other colors of mana, flying isn't that important, but an extra poison counter here or there is a huge deal. But this is true even for decks that aren't as all-in on single creatures. For example, this Zoo list plays Noble Hierarch and not Birds of Paradise. It has plenty of other ways to get the red mana, and even if sometimes it attacks with multiple creatures (missing out on exalted), the extra damage on the turns when it only has one creature to attack with can make a huge difference.