We can interpret this line in two different ways (although they ultimately both have the same end result).
In the first interpretation, we can perceive "put money in thy purse" to mean that Iago is instructing Roderigo to start saving up (or even generating more money, perhaps through selling land)...
so that Roderigo may go to Cyprus to win over Desdemona.
In the second interpretation, we can perceive this phrase as a common one used in the period in which Iago is essentially telling Roderigo that if he listens to Iago's advice, Roderigo can "bet on it" that he will win over Desdemona.
Either way, Iago is attempting to manipulate Roderigo by preying on his knowledge that Roderigo is in love with Desdemona. As the other instructor has mentioned, this situation ends quite poorly for Roderigo!