The first requirement is to complete the mission "Mister Walpole, I presume?" which is Sequence 2 
Memory 3 of the main story. Once completed and outside of a mission, open your pause menu (Esc key
for PC users), choose Progress Tracker, select Sequence 2, Memory 3, and start a replay of that 
same mission. Play the mission normally until you reach the point (after an obvious cut-scene) that
displays an optional objective to pickpocket the three Templars. Once at that point, pickpocket the
three Templars for an average of 750 - 850 Reales. Once pick-pocketed, immediately pause the game 
and choose Reload. Reload from last checkpoint. The game will allow you to keep the Reales you earned
and pickpocket the Templars again. Repeat the above steps until you have 100,000 or more Reales. 
This can be done in a few minutes time, and 100,000 can easily cover about 20% of the total game's
purchases. 
Note: The game developers prevent you from farming any more than 100,000 Reales on your person using
this tactic, so to maximize profit, don't begin farming until you've spent all your previous money to
get as much as you can out of it.