Edit: It has come to my attention that a patch has/will disable this method. It so please let me know. The fastest way to do it now is to sell all your stuff, reload, repeat.
There is an easy way to speed up earning this achievement. Money earned from selling back items adds to this achievement so if you purchase an item and immediately sell it back it will add to this achievement. You can keep purchasing and selling back the same item. By doing this with Farris, the treaty seller, in Skyhold near Blackwall. You can also earn the following achievement quickly.
To unlock Farris you must complete the war table mission: Power is Money.