In Othello, why does Iago want Roderigo to "put money in thy purse?"
Iago is an extremely clever villain. He wants to carry out his plan to ruin Othello by using other people to do his dirty work for him, thus avoiding the chance that someone might cast suspicion on him. He almost succeeds. He is so good at what he does that people even call him "honest Iago."
Roderigo is one of Iago's pawns. Iago knows that he can use Roderigo's desire for Desdemona to help him bring about Othello's demise, promising Roderigo that once Othello is out of the way, Desdemona will surely want him.
So, he tells Roderigo to save up his money (which is what he means when he says 'put money in thy purse') and follow the army to Cyprus. He thinks it will be easier to be rid of Othello when they are away from Venice, and he can use Roderigo as a scapegoat. Of course, this does not turn out well for the gullible Roderigo. He spends all of his money and ends up dead anyway, stabbed by his good pal Iago.
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!
"Put money in thy purse" could also be interpreted as Iago encouraging Roderigo to make more money so he can buy better gifts for Desdemona which Iago will ultimately end up having.