This achievement sounds tricky. The reason why I say this is because you can obtain 50,000 coins during your play through and you will not even notice since the achievement will not unlock that way.
In order to achieve this one, you will have to save your gold. You need to have 50,000 gold saved up and (Achievement unlocked)
I played through the game for a while and bought 3 guns and a horse. Don't forget all that ammo you purchase. I felt like saving money and check if the achievement will pop before 50,000 or I actually needed 50,000 in my wallet.
Achievement popped at 50,021 gold(In my wallet).... happy hunting.