As a hunter, I make a nice chunk of change leveling pets in Icecrown. Not only do those packs of champion guys (help me out here, I can't remember what they're called but they're for one of the Argent Tournament quests) give quite a bit of money themselves, they also drop frostweave cloth out the butt, not to mention tons of greens and even blues here and there. Just getting one pet from 75-80 often nets me at LEAST 100g, if not more, and it generally takes about an hour (if I'm slow). That's not including dailies.
There are plenty of ways to make money in this game. You can gather, you can do dailies, you can farm those "champion guys"..... Sure, it might take a little longer if you don't do dailies, but it's not like there is only one option. It all depends on how dedicated you choose to be. You can't blame Blizzard if you get bored or frustrated with dailies (I admit I'm bored of them too, but that's my problem). If you really put your nose to the grindstone, you can get loads of gold pretty quickly. It's just a matter of how much time and effort you choose to put into it. It's in no way unfair to any casual player. Getting gold does not require that you be in a hardcore raiding guild (in fact, raiding often causes a loss of gold, lol), or that you have the best gear. It's available to everyone. YOU have to decide, "Hey, I really want that mount, I'm going to do dailies for a couple weeks," and that's it. Just get out there and do it! If it takes a little longer, that's fine. But I don't think making mount speeds cost a couple hundred gold is reasonable, considering how easy it is to make money. It should be something you have to work towards, not just handed to you. And if an individual doesn't want to put the time required into obtaining said goal, then that's that individual's problem, not the game's, and not anyone else's.
So in conclusion, I feel that 5000g is a fair price for this new flight speed. I imagine that the 280% will come down in price to balance it out though. Maybe something between 1,000 and 3,000?
Edit: Saving up for my chopper is going to cost 12,500g at LEAST. It's going to take a looong time to save up for that. But you know what? That's fine. I know what I have to do, I just have to make myself do it. Not like I have to rely on a guild to get money, I can do it myself, so I have no reason to complain. Do I think it's a bit ridiculous that it costs that much? Yeah, a little

.....but to be honest, I'm fine with it. It's not an impossible goal, just a time-consuming one.

But if I want it bad enough, I know that I have the motivation to do what needs to be done.