Drysc has written a book about epic flying mount costs on the forums -- there aren't many goldsinks left in the game, but that 5000g you've got to pay for an epic flying mount is still there for sure (just ask anyone who doesn't have one).
He says that yes, the flying mount is definitely worth it (it makes playing the game much faster, since most of the time spent in the game is actually travel anyway), but also that actually obtaining an epic flyer was always meant to be optional -- only for players who want to "go the extra mile." Not sure how he can say both of those things at the same time, but there you go. He also says the main consideration in the cost wasn't any effect on the economy, but rather simply making sure that it took a certain amount of time for players to obtain all that gold -- they averaged how quickly players could pick up gold versus how long they wanted players to work for the mount, and arrived at 5000g.
There is some (kind of) good news on the horizon for players who don't want to spend all that money, however. If and/or when Blizzard releases a higher riding skill, Drysc says they'll probably drop the price on this one. Of course, that doesn't really tell us anything concrete -- with the onset of siege vehicles, there may be all kinds of changes to how riding and driving works in the game. Until then, keep grinding, because apparently the only thing that will get you flying fast is a whole lot of gold.