This achievement is earned by having 65,000 treasure on one Skylander at the one time. It does not count any money already spent. This will not likely happen during your normal playthrough, and will require additional runs. Fortunately, there is a wonderful way to get potentially infinite money once you beat the game.
After the credits end, you are returned to the deck of the Dread-Yacht. Go into the door up the stairs to the right, and make your way up to the Crane Deck. You'll see a very large pile of treasure, around 6000, on the ground. Collect it, and then quit to the dashboard. Do not save and quit the game. The money is saved to your Skylander instantly.
Back at the dashboard, create a copy of your save data on a USB stick. Load up the game using the USB save, and you'll be back on the ship deck. Go back up to the Crane Deck, and you'll see the money is there again. Collect it, dashboard, and reload from the USB. Repeat this until you amass 65,000 treasure and unlock the achievement.
A word of caution: Before attempting this, try to collect all of the Winged Sapphires that can appear randomly around the ship. One can spawn on the Crane Deck, and if you grab it while getting the treasure, the game will save, and you'll lose your infinite money cheat.
Here is a video by Toxiicxducky831 showing the process:
@HimiTomoki One figure has to have 65,000 treasure - if you've played the first game and have figures with loads of treasure on anyway (I have a Spyro with about 40,000 treasure) then it should be easy.
Just keep doing arena challenges. It soon racks the coins up. Also on some levels enemies drop quite alot of treasure. Mainly later levels. If you have to complete every level again on nightmare mode to get that achievement then you should easily get one skylander to have 65000 treasure at one time. And yeah money does come over with skylanders from the first game
Once you finish the game you can go up to the crows nest of flynns ship and collect heaps of treasure. Then,if you dont touch the butterfly thing, you can re start from your last save and grab the loot again and again until you have enough. Dont touch the butterfly or the game will auto save.
As awesomenaut777 says, you can grab 6-8k worth of gems on the rear deck of the Dread Yacht after you beat Kaos. If the Winged Sapphire is there and you touch it, it will autosave, which is bad...but the Winged Sapphire shows up after Chapter 14, so grab it before you head into Chapter 15 or 16. Then beat Kaos, then head up to the back of the ship. Go up the stairs, above the door with the vault/store/skystones, and there should be a ton of gems there. Hit your Xbox button, exit to dashboard, then reload the game. You can also only hit the Xbox button, click X to sign out, then B to return to the game and it will kick you to the start screen and make you sign in again. This is a slightly faster process. I also copied my games saves from Xbox cloud to local memory card and hard drive and do