Edit 2/8/17: Some people are saying that this achievement isn't unlocking for them even after spending over $500,000. It's entirely possible that a recent update glitched it. If you're having trouble unlocking this achievement, please let me know in the comments, especially if you spent more than $500k in a single playthrough.
Edit 3/27/17: This may be fixed. If you haven't been able to get it to unlock before, try loading up the game to see if it will now work for you.
Edit 7/30/17: I don't know if spending money upgrading the marijuana grow op in Faster Baby! or Sammy's in Sign Of The Times counts toward this. However, it seems like a potential way to kill two birds with one stone, as there's achievements for fully upgrading both businesses.
Buying weapons and upgrades is the most obvious way to get this other than spending a lot of money on ammo, throwables, and armor. Since blowing through all your ammo and buying it up again just to spend money is stupid, spend your money on new weapons and upgrades for Lincoln and his arsenal.
You can't upgrade the special weapons, but upgrades for pistols, shotguns, rifles, and automatic weapons can be purchased for as high as $18,000. Upgrade Lincoln's accuracy, total ammo carrying capacity, stability, and reload speed. Note that you have to Cassandra's earn up high enough to purchase these upgrades from the arms dealer. The requirements are:
Improved Accuracy - $60,000
Increased Ammo Capacity - $140,000
Improved Stability - $220,000
Faster Reload - $370,000
This achievement isn't too difficult as long as you're maximizing the earn from all of your rackets and you're well into the game. I was buying some of the weapons and all of the upgrades as they became available and still had $100,000+ in the bank at all times.
Getting Vito's earn to $140,000 upgrades the Consigliere Assistant so that she will automatically collect your kickback from your Underbosses. This saves you a lot of driving and keeps the cash flowing in.
(It should be noted that progress on this achievement carries across playthroughs. And since this game requires at least two playthroughs to 100%, if you're not in a hurry you could just shoot for $250k spending per playthrough which would be very easy.)