You can easily earn around 50k per hour grinding the Race at Los Arcos. The final round pays 840 gold and takes less than a minute. I was able to win using the English Bloodhorse. If the English Bloodhorse is out of your price range you can get to the final round of the Socorro race with a Spanish Mustang. The final round pays 750 and the race is a bit longer. The Eagle Eye mission can potentially offer better returns if you are really good at it. The final round pays 750 and can be finished in a mere 15 seconds. Also this achievement does not unlock when you have cumulatively earned 50k, but rather you must actually have that amount in inventory.