Saving for a Rainy Day

Raiding is the fastest way to earn this achievement. You get gold that counts towards this achievement not only from what you loot, but from what people in your party/raid loot as well. So although of course Cataclysm raids would be best, LK, BC and Classic raids also work. Some outdoor raid bosses are good to farm for this as well. Some 5 man dungeons are okay, especially on heroic. And obviously, the fewer people you bring, the more gold for everyone.

Basically, it's a tossup about whether you want to do higher-level content with a bigger group or do lower-level content with a smaller group (and perhaps even solo it.) It's up to you to pick the content you want to do and with how many people. There's a lot of raids and dungeons in the game to choose from.

Just make sure you and your party/raid members are looting every corpse! It often happens in raids, especially large ones, that members don't loot their corpses. A corpse will have sparkles if there are items to be rolled on there, and the person that loots will be able to loot the gold and view the roll-able items; any other non-rolling items will be hidden and only lootable by the person the loot is assigned to. This means if a corpse has no roll-able items, only the person who is assigned to loot it can get the gold (which would get split to everyone and count towards achievements.) If they don't loot it, no one gets it!

Raid bosses always have the most gold, (All these bosses are worth 100g or more!), but the constant looting of trash along the way definitely contributes to the achievement. While I always run my raids on the 'need before greed' setting, perhaps changing the loot to 'free for all' for trash would help make sure every corpse gets looted, since you can then loot them all yourself if you have to.

You can actually make a fair bit of gold this way, as long as you don't fail and wipe a lot. Between vendoring all the epics you don't need (or disenchanting and putting mats on AH), selling the occasional desired BoE on the AH, and all the gold you get as you loot along the way, I made more gold than I thought I would. Doing 'old raids' is a way to make money after you've done your dailies every day, and it's usually a little more interesting than simply farming. Some rings and staffs and 2 handed weapons from LK raids can vendor for up to 20-30 gold a pop! Still, there's much faster ways to simply make gold, except that most of them don't count for this achievement. If this achievement is what you want, get raiding. =P

You can work towards this achievement by:
Looting cash from monsters yourself. (Vendoring grays or other items does not count.)
Looting cash from openable items that create a loot window. (Here's some worth 1 gold or more a pop.)
Looting cash from enemy player corpses in battlegrounds.
Receiving cash from when a raid/party member loots cash from monsters.
Pickpocketing cash from monsters if you're a Rogue.

My guild just got this achievement. We got the achievement from the Bag of Coins that drops in Zul'Aman.
NOT actually looting a monster. (I know this because i finished the achievement and was the only one online at the time)

My guild just got the 100k gold achievement, and we raid regularly, not 'hardcore' though. In my opinion this achievement should be looked at by most guilds as 'we will get it eventually (but not terribly long from now) through normal play). I think they should add a loot 500k gold achievement, that one would seem much less trivial, in my opinion.