For the purpose of our discussion today, I'm classifying anything that costs 1,000 gold or up as a reasonably expensive purchase. While there are other achievements that will probably cost you a great deal of gold to get (Working Day and Night comes to mind if you've power-leveled professions like Jewelcrafting or Inscription), how much you spend on them is usually subject to how quickly you want to do them. By contrast, the achievements below are entirely dependent on your ability to amass a huge pile of gold and then use forklifts to distribute it to NPCs in return for achievement points.
And if you want all of them? The sum total of the 6 achievements below will probably run you somewhere in the region of 48,000 to 62,000 gold.
Grand Ice Mammoth
At exalted, the Sons of Hodir will sell you a three-person mammoth that is apparently really expensive because it comes in an attractive white hue. Well, that, and it's also the only three-person mount that isn't a drop (like the Grand Black War Mammoth so beloved by ninjas) or an even more absurdly expensive purchase (the Traveler's Tundra Mammoth, see below). Mammoth mounts on their own needn't be overly expensive (e.g. Black War Mammoth) if you really want an annoyingly large mount with which to block doorways and clog the Dalaran fountain. But passenger seats? They're gonna cost you.
Damage: 8,000 gold at exalted with Sons of Hodir.
Dual Talent Specialization
These days, dual-spec is widely considered a must-have addition to gameplay if your main character's a hybrid, does any amount of PvP, or just has good reason to switch specs in PvE a lot. If this describes you, then the up-front cost for buying dual-spec will almost certainly pay for itself before too long, but it's still a decent amount of money to drop for the feature.
Damage: 1,000 gold.
Cenarion War Hippogryph
For this baby, you'll need to be exalted with the Cenarion Expedition in Zangarmarsh in addition to a decent chunk of cash, but in return you'll get the first of what are now three hippogryph mounts available to players (two, sadly, if you're Horde). I am utterly in love with their animations (particularly because they can now land, run, walk, and have a cool /mountspecial that...looks familiar!) but am dearly hoping that Blizzard updates their graphics. Thanks to our reader Jon Do for pointing out this one in the comments.
Damage: 1,600 gold.
Get to the Choppa!
It has to be said that getting a Mekgineer's Chopper (Alliance) or Mechano-hog (Horde) is theoretically free if you find a very generous Engineer, but the odds of this happening are slightly less likely than your correspondence with a dying Nigerian prince via email resulting in a million-dollar wire transfer. Some of the materials for the mount (namely, the Salvaged Iron Golem Parts, the Goblin-machined Piston, and the Elementium-plated Exhaust Pipe) can only be obtained for a very high fixed price from a goblin NPC in K3, or else "skinned" off mechanical mobs in Ulduar by Engineers.
Damage: This will vary by server and whether or not you've got an Engineering toon with easy access to Ulduar, but you should realistically expect to shell out between 13,000 to 15,000 gold on average.
Ring of the Kirin Tor
The good citizens of Dalaran have become adept at soaking players for massive quantities of gold and badges, and the NPCs in the jewelry shop are no exception. The four versions of the Kirin Tor rings are actually decent options for their various i-levels (200, 213, 226, and 251), and the latest incarnations are a match for the gear you can get from ICC-10. They also come with an oft-useful teleport to Dalaran on a 30-minute cooldown, although that's obviously much more useful for some classes than others.
Damage: Assuming you're exalted with the Kirin Tor (and you should be before you purchase a ring), 6,800 gold.
My Sack is "Gigantique"
Interesting name, eh. The Gigantique Bag sold by Haris Pilton in Shattrath's Lower City will run you a cool 1,200 gold, but unlike most of the game's larger bags, bears the distinction of being BoE and not unique-equip. With a nice pool of gold you can equip yourself and your alts in a full kit of 22-slot bags (unless you choose to go for Pilton's newest bag, the Portable Hole, which affords you 2 more slots per bag but no achievement). Thanks to our reader Rob for reminding me about this one!
Damage: 1,200 gold.
The Right Stuff
The fabled Artisan riding skill, otherwise known as epic flight, is one of the game's greatest time-savers -- but it won't come cheap. The cost of epic flight has been the target of many, many player complaints since its introduction, but Blizzard is unlikely to budge. For many people, the problem with Artisan riding is that it's not a one-time deal; getting epic flight on multiple alts is a tall order for the average player. Still, for any toon on whom you plan to put in a lot of time questing or gathering, the answer to "Is epic flight worth it?" is an emphatic yes.
Damage: The base cost is 5,000 gold, but this is now subject to your character's reputation with either Stormwind/Alliance Vanguard (Alliance) or Orgrimmar/Warsong Offensive (Horde). Assuming your character is exalted, you can (and should) knock the price down to 4,000 gold.
Traveler's Tundra Mammoth
For hardcore farmers and dungeon-runners everywhere, the Traveler's Tundra Mammoth -- equipped with two vendors, one of whom can also repair your gear, and both of whom you can eject from the mount in a rather entertaining fashion while the mount is in motion -- is a huge convenience. Otherwise, whether or not you'll think it's worth it depends a lot on what you do within the game. I've found it useful as both a quick repair option in raids where it's possible to mount, and -- while farming older dungeons and raids for reputation and drops -- as a significantly faster means of emptying my packs without having to run to the nearest town. Assuming that Blizzard continues to update its vendors in future expansions to accommodate higher-level food, drinks, reagents, and ammunition, consider the mammoth a very large investment that will slowly but surely pay for itself.
Damage: As with the rings, you should plan on being exalted with the Kirin Tor before purchasing this baby, and it'll run you 16,000 gold.