The description is self-explanatory--the achievement unlocks when your cumulative expenditures of all types reach 200K gold.
At the beginning of the game, gold will be tight and you will not want to squander it. Initially, you will spend your gold on necessities like minor healing potions and repair kits. They are inexpensive so your best use of leftover funds is to purchase the services of trainers. Each trainer will boost your skill in the area of their expertise by a point. Basic trainers (levels 0-3) charge 15,120 gold. Advanced trainers (levels 4-6) charge 30,240 gold. Master trainers (levels 7-9) charge 45,360 gold. If you pay attention to the location of trainers (small sword icon on mini- and local maps) especially in Star Camp, Ysa, Rathir, and Adessa that each have more than one trainer; you will quickly spend over 200K gold on trainers alone and the achievement will pop.
Alternatively, if you buy only what you need and don't want to seek out trainers, you can hoard your money. When you reach Gallows End in Legend of the Dead Kel, you'll come across merchants that stock items that cost over 200K gold apiece. Make a save before dealing with the merchant. Buy the expensive item. When the achievement pops, reload your save and continue on none the poorer. Gallows End is not the only place that sells expensive items. Later in the game, you'll have more gold than you know what to do with and you'll find merchants that will sell weapons, armor, and accessories at exorbitant prices.